The Discovery of the Trinity
Two basic tenets of Catholic teaching are that 1) God revealed himself in a progressive revelation that was completed with the death of the last apostle and 2) since then the Church's understanding of that complete revelation has deepened and developed.
Perhaps the classic model for understanding this process is seen in the revelation given by God concerning His own Triune nature. Certain critics of the Catholic Faith speak of the doctrine of the Trinity as an "invention" of the Church. However, it is closer to the mark to say that this truth was discovered rather than invented. For the Church, so far from creating anything, simply followed the clues left by God in His complete revelation given through Scripture and Tradition.
The clues were essentially as follows:
There is but one God. This is the theme drummed into Israel by the tradition and Scripture of both the Law and the Prophets. "Hear O Israel! The Lord your God, the Lord is One!" (Dt 6:4) is the very heart and soul of the Old Testament. Alone among all the ancient nations of the earth, Israel is chosen by God to be the one nation in covenant with this one God. Alone among the nations of the earth, Israel is held to fidelity to this one God through disaster, enslavement, deportation, conquest and political humiliation. That there is one God is the unshakable revelation given to the Jews.
However, something else is given to the Jews: the promise of a Messiah. What this Messiah will look like is up for grabs at first: Healer, Bringer of Peace, Conquering Davidic King, Suffering Servant. All these hints roil about in the mix--until Jesus appears and unites them all in His person. But He does something more than that. While repeating the refrain of all ancient Israel that God is One, He also forgives sin, which prompts the Pharisees (and his disciples) to ask "Who can forgive sin but God alone?" (Mk 2:7) He calls himself the Son of David, but also implies that he is "David's Lord" (Mk 12:35-37). He names himself "I AM": the very name of God (Ex 3:14; Jn 8:58). In short, He claims to be the Lord of the Universe Who led Israel through wilderness, gave the Law, and called the Prophets. He is also, by His own account, somehow distinct from the One He calls "my Father, who is greater than I" (Jn 14:28). Yet at the same time, He insists "I and the Father are one" (Jn 10:30). Moreover, he teaches that there is yet Another, a Paraclete, a Spirit of Truth, who proceeds from the Father (Jn 14:16-17; 15:26). And He commands that His disciples baptize in the name of the Father, Son and Holy Spirit (Mt 28:19).
This is the basic problem set by the Christian revelation. The faith of the Church is in one respect identical to Israel's: There is but one God. But it also cognizant of further revelation, summed up in Simon Peter's declaration: "You are the Christ, the Son of the Living God" (Mt 16:16) And it is further complicated by the fact that both Jesus and His apostles teach that the Spirit (or Holy Spirit or Spirit of Jesus or Spirit of Truth or Advocate) is also a personal being who convicts of sin, gives graces, enlightens and guides the Church into all truth.
How then does the Church piece together this mysterious revelation? Very slowly, and with a conscious reliance on the Spirit of Truth to, in fact, do what Christ promised and guide the Church into all truth (Jn 16:13).
Various attempts to reconcile the data are proposed by various early thinkers. Some have certain insights to the Truth (for example, it is a second century Christian (Tertullian) who coins the term "Trinity", yet also certain failings (nearly always due to the fact that these early Christian thinkers try to make certain data "fit" by suppressing or ignoring other data). Thus, for instance, some put forward the notion that God is one by denying that the three Persons are distinct. Others attempt to emphasize the distinctness of the persons and wind up advocating something that looks like polytheism. Still others attempt to float a theory that the Old Testament God is bad while Jesus is the Good God of the New Testament, come to rescue us from the Bad One.
All of these theories are weighed and sifted in the Church for nearly three hundred years and the Church continually states definitions of what she does not believe till finally a theory appears which seems (like the devil tempting Christ) to explain all the biblical data and yet which strikes at the very heart of the Church's Faith: Arianism.
Arius developed the novel notion that Jesus was a supernatural created being. Christ was, said Arius, vastly superior to us (as an angel is) but still created and not of one being with the Father. Arius argued that various Scriptures (such as "I and the Father are one") referred to the oneness of Jesus' will with God, not the oneness His being. And since Jesus was created, according to Arius, the logical consequence was that worshipping Him as we worship God was, in fact, a sin. This "simple" theory, while appearing to be faithful to the Oneness of God, also completely destroyed the preaching of the Church that Jesus was literally "God with us." If Arius was to be believed, then it meant that Jesus' death, like the death of any other mere creature, could neither save from sin, nor bestow on us what the apostles had promised: a participation in the divine nature (2 Peter 1:4). For even Jesus cannot give what he does not have.
How did the Church respond? It assembled in Council first at Nicaea and later at Constantinople. At these Councils, the Church reasserted the traditional understanding of Scripture that God was indeed one (as Arius insisted) but that this oneness was a oneness of union between the Persons of the One Godhead, not a oneness of isolation. The Councils reaffirmed not only that the Word was with God (as Arius taught) but that the Word was God (as John 1:1 taught). In so doing, the Church made a historic step. They delineated, not merely what they did not believe about the Godhead (as was hitherto the case in questions on this matter), but what they did believe. They chose a series of careful statements (God from God, Light from Light, True God from True God, begotten, not made, one in being with the Father) that summarized not only all the Church had rejected in her thinking, but what she positively asserted in the face of the various attempts to suppress portions of the biblical data in favor of false "simplicity."
In short, nothing was invented by the Church with respect to the Trinity. Rather, the Church sought to prevent a "simplifying" invention by Arius and remain true to all the biblical data, not just pieces of it that Arius liked. Paradoxically, in fighting the invention, the Church discovered a far deeper understanding of what she had always believed and formulated it in Nicene Creed.