The Priesthood is one of the 7 Sacraments of the Church.

First, I think it's important to understand that the priesthood as such or the notion of the priesthood, has a foundation in nature. Even in natural religion without revelation or even without Original Sin we would still have required a priesthood for the public worship which would be offered up to God.

However, Our Lords priesthood is something Supernatural, but to prepare us for this priesthood, he called forth men before his coming to exercise the office of priests, which would be only an image, or feeble representation of the priesthood of Our Lord Jesus Christ. I say the Priesthood of Our Lord Jesus Christ because every Priest of the New Law is only a valid priest in so far as he participates in the priesthood of our Lord Jesus Christ.  And so to best understand the priesthood I would say we would have to go to its source - The Priesthood of Our Lord Himself, who as St. Paul states is a priest forever according to the order of Melchisedech" (Hebrews).

Before we look at the Priesthood of Christ itself, it would be of advantage to take a step back and ask ourselves why He even became a Priest at all? 

The answer lies in the fact that after the fall all man kind has been plunged into the state of perdition. The state of the “Massa Damnata” (St. Augustine) . After the fall if Christ out of obedience to do Gods will and redeem us we would have only been destined for Hell. You only purpose on life would have been to propagate, die and go to Hell. If you understand that, you will grasp truly the importance of the Priesthood of Christ. It is in many respects this idea, which St. Paul tries to explain to the Hebrews. 

And so we see that Our Lords priesthood has a direct relation for our own salvation.
 

The Priesthood of Our Lord is a doctrine de fide by the Church and proclaimed by the Council of Trent (Session 22, Chapter 1). St. Paul explicates the priesthood of our lord in the words "Christ did not glorify Himself that He might be made high-priest. He received this glory from Him who said "Thou art my Son, today have I begotten thee" (Hebrews) .

While its explicit from scripture that Christ is a priest, what perhaps is not so explicit is how did Our Lord become a priest or how was he Ordained? Well Our Catholic faith tells us that Christ was a Priest by virtue of His Incarnation by which his was united to the Person of the word of God. In Other words, his consecration took place in the greatest basilica - the womb of the Blessed Virgin Mary, in which he was anointed by the Holy Ghost with the Divine Chrism, (while the Blessed Virgin Mary supplied the matter).  This great day of Christ ordination, upon which the salvation of the world depended, is recorded in the first Chapter of St. Luke’s Gospel, where the Angel Gabriel announces to Our Lady that (Luke 1:35)  “The Holy Ghost shall come upon thee and the power of the Most High shall overshadow thee.” To which Our Lady responds with those famous words, which still echo throughout human History “be it done to me according to thy word”.  At this moment in an instant, the word of God was made flesh and Our Lord became the High Priest of the New Law for the redemption of the world.

From the very moment of His incarnation Our Lord began to act as Priest by praying and interceding for us to the Father. As he grew in stature and age He began to exercise more and more explicitly His priestly function by  communicating to men the sublime truths of the faith and by establishing the great means of salvation - the 7 sacraments. Yet all His priestly functions were perfectly summed up by the offering of Himself on the altar of the cross as perfect offering to God for our redemption in order to make an atonement for our sins and show forth his perfect love, obedience and resignation to the will of God so that we may see His great love for the Father and His infinite love for souls for whom he died.
 

Christ was both True God and True man and so he could offer something to God on our behalf which would be of an infinite value while obtaining for us, Our redemption and Gods grace and Mercy so that we are no longer strangers to heaven but that with Christ merits we can now say that in a certain way as Christians we have a right to heaven as adopted sons of God.
 

What is most striking about the priesthood of Christ is that in offering His sacrifice on Calvary, Christ was both priest and the victim, He was both the one who offered the gift to God, while also being the gift that was offered. In all previous sacrifices offered by a priest, the priest and the thing he offered up to God were two distinct things. In the New Testament Priesthood, Christ reconciles these two things in himself. This reality is what should elevate our love for the mass even more since in every mass offered by the priest today, Christ is here and now the principal offerer (Dz 940), which is why the priest says at mass “this is my body and my blood” and  not “this is Christ’s body and Christ’s blood”. Since it is really and truly Christ who is actually exercising his priestly office through the priest.

So in other words the Eternal Son of God comes down from heaven as a great expression of his love for the father and in faithful submission to God offers His priestly life as an offering for the redemption of the world. – “God so Loved the world that he gave us his only begotten son that those who believe in him might not perish but have life everlasting” (John 3:16).
 

We are truly in need of Christ priest hood. It is essential necessary for our Salvation, so much so that without it we would simply be “Massa Damnata” – A mass of People destined only for eternal torments in Hell. In fact that is why even when we concede the efficacy of the rites of the Old Testament in being able to confer grace on the Jews we unhesitantly  affirm with St. Thomas that it was only so in virtue of the Sacrifice of Christ. In other words its because as St. Thomas points out, the Jews of the Old Testament ultimately had the same object of Belief as Catholics of the New Testament (i.e the belief of salvation by the Messiah to come), that their ceremonies derived there efficacy. The Jewish rites were only a prefigurement or shadow of the reality of the redemption that was to be brought about by the Son of God.  That is why St. Paul comes down so hard on the Jews and affirms to them that the Old Law is a dead letter without Christ since  its whole intention was to prepare them for the Messiah who was to come. So now when they read the law it’s only with a veil over their eyes (2 Cor 3).

This necessity of Christ priesthood allows us to have a clear and practical expression of the dignity of the priesthood itself (Summa Theologica IIIa, Q. xxii, Art. ii) since it allows us to understand that the celebrant at the altar is only the minister of Christ and not his successor. And so the sacrifice of Calvary and the sacrifice of the Mass are essentially the same sacrifice, the same victim and the same principle offerer - differing only in the manner of offering. On Calvary there was suffering, shedding of blood and merit, now the sacrifice is bloodless and sacramental. No longer is it painful and meritorious but confers on us of its very nature (ex opere operato) the satisfaction and merit of the passion and produces in our souls the abundance of grace according to our dispositions.
 

Hence the ultimate reason Christ instituted His priesthood was to make of men, other Christ's, (alter Christus) so as to continue his work of redemption by giving them the power to participate in His own priesthood so that may continue to be His voice in this whole “massa damnata” (damned mass of people). That is why Christ makes it clear to His apostles “ He that hears  you hears me: and he that despises  you despises me” (John 10:16) . These words in fact loose their meaning unless their is a Church or a Priesthood. Christ doesn’t say, he who despises the bible, but “he who despises you”, to indicate that they will be His very own living witnesses in and amongst an unbelieving world.
 

The Council of Trent declares that at the same time as Christ instituted the Blessed Sacrament he also instituted the priesthood of the New Law (Luke 22:24) by the words spoken after the consecration of the bread and wine "Do this for a commemoration of me" . By theses words Christ ordained the Apostles to the priesthood for offering the sacrifice of the Mass. The Church affirms this because with the command to do something also comes the power to do it.

After his resurrection, Christ also gave the apostles the power of forgiving sin when he breathed upon them saying "Receive the Holy Ghost; who soever's sins you forgive they are forgiven, who soever's sins you retain, they are retained" (20:21-23). So now the Priest forgives sins not in his own name but in the name of Christ.

While the word “priest” is continuously used throughout the Old and the New Testament, it still nevertheless contended by some that the word is not found in the bible. The problem with such an argument is that it often indicates ignorance of any real biblical theology since the word “Priest” in the English is derived from the Greek, the word for elder; “presbuteros” which is used throughout the New Testament . In Latin, the word is “presbyter”, which in English became shortened to the word “priest”. That's why you never hear about "Catholic elders." It is because Catholic priests are Catholics elders. Any standard  dictionary confirms this fact.

Since Our Lord instituted the sacrament of Sacred Orders as the means to communicate the reality of His incarnation and redemption. The Apostles in obedience to that call simply continued the rite of ordination by mean of the laying on of Hands and praying over the candidate, which is what we read in St. Paul letter to Timothy as he states : "Neglect not the grace that is in thee , which was given thee by prophecy, with imposition of the hands of priesthood" (I Tim. 4:14). We notice here how the Holy Spirit was given: "...with imposition of the hands of the priesthood."

The “imposition of hands” which St. Paul refers to is today seen as indispensable for conferring valid orders since it is not enough to merely be appointed by the people or by the elders, since the candidate still have to be ordained to the priesthood “by the imposition of hand". In fact St. Paul emphasis this to Titus as he states "For this cause I left thee in Crete, that thou should set in order the things that are wanting, and should ordain priests in every city, as I also appointed thee:... (Titus 1:5, cf also I Tim. 22).

And so to these Ordained men alone who have been lawfully ordained belongs  the office of ruling the Church, of dispensing the mysteries of God, and of offering the Holy Sacrifices of the Mass (Acts 6:6, 14:22, 20:28, 1 Tim 4:14, 2 Tim 1:6, 1 Cor 4:1, Heb 5:1). What is more is that that those who serve as leaders of the Church- the bishops-are do so, by a divine right as is expressed in Paul's command to the  Macedonian Catholic bishops:

"Take heed to yourselves, and to the whole flock, wherein the Holy Ghost hath placed you bishops, to rule the church of God, which he hath purchased with his own blood (Acts 20:28).
 

What I would like to stress in this conference is the great immensity of Power our Lord did leave to his priests and in particular to His bishops - Just listen to what he says to His Apostle after the resurrection : "As the Father hath sent me So all so I send you" (John 20:21). Now if all power in heaven and on earth has been given to Christ and Christ himself had commission his apostles to continue his same mission. So the Priests and Bishops are given a real physical power and not a mere delegation. They exercise their authority, as true ambassadors of Christ, St. Paul explicates the point saying “For Christ therefore we are ambassadors" (1 Corinth 4:1).
 

Along with the indelible character comes the special sacramental grace which follows every sacrament since God never confers on any man a power without giving him the necessary means for the worthy exercise of that power - the same is true of all the sacraments - and so sacramental grace is given to the  priest so that he can perform the duties of his state not merely validly but also holily, and that and that this sprit of holiness, which is intended to characterize all his work , may develop in unison with his growth in grace and charity which is demanded by his vocation . The Sacramental Character of the Priesthood like that of Baptism and confirmation, will endure even after this life, in the good and faithful priests it will redound to their glory; but for the wicked and faithless priest it will exemplify their disgrace so that in hell he will be a special instrument of the devils torments.
 

 Before dealing with the issue of Priestly celibacy I would like to address a minor point which I think is important: the priesthood of the faithful - It's a point that is so often misunderstood by Catholics and by non- Catholics who deny the ministerial priesthood as instituted by Christ.
 

This notion might be a surprise to you but the notion of universal priesthood does have its place in Catholic Theology. The Problem is that non-Catholics will always bring up the words of St. Peter " You are royal priesthood."- In order to do away with the ministerial priesthood. However, what they forget is that this notion of a "universal priesthood" of all believers is not something new. St. Peter was merely quoting from the book of Exodus were God said to Moses "thus shall you say...to the people of Israel...now...you shall be to me a kingdom of priests and a holy nation." (Exodus 19:3,6) .
 

Now if this was understood in the Old Testament were they still maintained a distinct ministerial priesthood from that of the common priesthood of all believers, how much more should this distinction so exist in the New Testament since as I previously mentioned the Old Testament priesthood is far inferior to that of the New.
 

And so, we see that in biblical theology, the three-fold model of the priesthood which was in use at the time of Aaron in the Old Testament  was carried over into the New Testament and thus we find there also a high priest, ministerial priests, and the common priesthood of all believers. In the New Testament age the high priest is Jesus Christ (Heb. 3:1), the ministerial priests are Christ's ordained ministers of the gospel (Rom. 15:16), and the universal priests are the entire Christian faithful (1 Peter. 2:5, 9).
 

Our Holy Faith  tells us that all the faithful participate in the priesthood of Our Lord in a passive way- that is they now have the power to receive the sacraments due to their character of baptism and confirmation, without which they could not partake of the other sacraments. As to sacrifice that they offer, St. Peter tells us that they are to offer up "spiritual sacrifice acceptable to God by Jesus Christ" (1 Peter 2:5). In other words the faithful  offer up spiritual sacrifice on the altar of their hearts to God. This priesthood however is what the Council of Trent refers to as the "internal Priesthood" of the baptized faithful. While the Ministerial priesthood of the Ordained, alone has an external active participation in the priesthood of Christ by which they offer up not merely a spiritual sacrifice but a substantive sacrifice – The Holy sacrifice of the Mass.

Spiritual fatherhood

Peter followed the same custom, referring to Mark as his son: "She who is at Babylon, who is likewise chosen, sends you greetings; and so does my son Mark" (1 Pet. 5:13). The apostles sometimes referred to entire churches under their care as their children. Paul writes, "Here for the third time I am ready to come to you. And I will not be a burden, for I seek not what is yours but you; for children ought not to lay up for their parents, but parents for their children" (2 Cor. 12:14); and, "My little children, with whom I am again in travail until Christ be formed in you!" (Gal. 4:19).

John said, "My little children, I am writing this to you so that you may not sin; but if any one does sin, we have an advocate with the Father, Jesus Christ the righteous" (1 John 2:1), "No greater joy can I have than this, to hear that my children follow the truth" (3 John 4). In fact, John also addresses men in his congregations as "fathers" (1 John 2:13

By referring to these people as their spiritual sons and spiritual children, Peter and Paul and John implicitly refer to themselves as their spiritual fathers. Since the Bible frequently speaks of this spiritual fatherhood, we Catholics acknowledge it and follow the custom of the apostles by calling priests "father." Failure to acknowledge this in fact is a failure to recognize and honor a great gift God has bestowed on the Church: the spiritual fatherhood of the priesthood.

Catholics have great filial affection for priests and call them "father," knowing that as members of their parishes they have been committed to their spiritual care since to them the priest ought to be the living embodiment of Christ Himself. It is for this reason that the Church has always had a high ideal of the priesthood and the duties and functions of the priest.