True Religion
I posit a question. “Can there be more than one true religion?” To answer, it would be a good idea to talk a bit about religion itself. Religion is a virtue that is related to the virtue of justice. We recall that justice is the virtue in which we render what is our due. Religion is the virtue of justice applied to God, which is to say it is rendering to God what is due to Him.
In the natural order, it is possible to know that there is only one God and that He is not me. It is possible to recognize that the world is fallen and that God punishes the guilty and rewards the virtuous. The natural law being written on the heart of man, it also follows that, in the natural order, one can conclude that it is one’s duty to do good and avoid evil. The virtuous pagans recognized as much. It is further possible to recognize that all a man has and is comes from God; thus, in justice (religion), we owe all to God.
For us to know about God beyond what we can know from nature, God had to reveal Himself. We call all of that which God has revealed to us Divine Revelation, and we believe with the gift of faith that this body of knowledge is true because God has revealed it.
Returning to the original question, logically, we must answer in the negative. There can only be one true Faith, for the truth about God is complete and not contradictory. God did not reveal falsehoods about Himself. Likewise, there can only be one true religion. The true religion is that one which renders back to God that which God wants rendered and in the way God wants it rendered. Deviate from this, and you no longer have true religion. You might retain bits and pieces of the true religion, but you will not have true religion.
The Synagogue
In the Old Testament, we read how God chose to reveal Himself to a nation (the Hebrews) and how He established laws for priests, for sacrifice, and for the people, and how he promised to bless the world through them by the coming of the Messiah. We see examples (such as Aaron’s sons burning unlawful fire or Saul offering sacrifice in disobedience) where men chose their own way and did not render to God as God desired. We see more blatant examples (such as the Hebrews worshipping the golden calf) where a large faction of the people abandoned the true religion.
God established a covenant with Abraham and with the Hebrew people. He would be their God, and they His people. He established the Jewish religion, and the blood of the covenant was that of animals. When the Messiah did come, however, His people did not accept Him. Rather, they chose to kill Him, and in doing so ended the covenant. (A covenant, like a marriage is good until either party dies, and God died.)
I ran across the following: “The Old Testament is an indispensable part of Sacred Scripture. Its books are divinely inspired and retain a permanent value, for the Old Covenant has never been revoked” (Catechism of the Catholic Church 121).
Unfortunately, the Catechism does not elaborate on this last phrase, nor does it speak of any implication other than that the “[Old Testament’s] books are divinely inspired and retain a permanent value.” (That was never a question in my mind.)
So that there is no confusion, my source for what I wrote was what I heard in a homily. The priest may have been mistaken in the terms he chose. Nevertheless, I think the priest’s opinion can be reconciled with the Catechism in the fact that today, our path to salvation is through baptism, the Old Covenant having been fulfilled by Jesus Christ. We see after Pentecost that the Jews who converted were baptized and that this was necessary. Likewise, the circumcision of the Old Covenant was no longer required. If the Church is teaching “the Old Covenant has never been revoked” then I accept that; I just don’t know entirely what that means.
The Church
With the coming of the Messiah, God visited his people. They rejected Him, and He established the New Covenant in His Own Blood. The Jewish religion no longer rendered to God according to His will; for it rejected Divine Revelation (in Christ revealing Himself) and its sacrifices were no longer accepted by God (hence the veil of the temple being rent in two). In fact, the Jewish religion, which to that point had been the true religion, ceased to be the true religion.
Where then was the true religion? The true religion came from Christ Himself. We read in the Gospels how He promised to found a Church. We read how, after Christ’s ascension, the apostles were the recognized leaders of that Church with St. Peter supreme among them. We read how Christ promised to remain with that Church until the consummation of the world. The priests of that Church offer the acceptable sacrifice. That Church is the true religion, rendering to God that which God has commanded and maintaining the deposit of Faith. It is the One, Holy, Catholic, and Apostolic Church.
The True Church is one: it is not a collection of various, contradictory faiths. It proclaims one faith only.
The True Church is holy: it is set apart for the authentic worship of God, and it sanctifies men.
The True Church is Catholic: it is universal, available not just to a few but to all men.
The True Church is apostolic: it has been passed on from the Apostles and can be traced back to them, who were the eyewitnesses of Christ and the founding of the Church. Put another way, every Catholic priest was ordained by someone who was ordained by someone who, ultimately, was ordained by one of the twelve Apostles, who were ordained by Christ.
Other Churches
Since Christ founded the Church, various sects have risen and died out. Many heresies have sprung up, eventually to retreat into the annals of history. With the Protestant Reformation, however, the number of Christian denominations has exploded into the thousands, though most have their roots among the Lutherans, Calvinists, Baptists, and Anglicans. These denominations cannot all be the true religion, although each may contain some elements of the truth. In particular, none of them can claim to be apostolic.