In what year was the Trinity accepted as doctrine in the Roman Catholic Church?
In the article some references to the Old Testament too.
The Trinity is the term employed to signify the central doctrine of the Christian religion — the truth that in the unity of the Godhead there are Three Persons, the Father, the Son, and the Holy Spirit, these Three Persons being truly distinct one from another.
Thus, in the words of the Athanasian Creed: "the Father is God, the Son is God, and the Holy Spirit is God, and yet there are not three Gods but one God." In this Trinity of Persons the Son is begotten of the Father by an eternal generation, and the Holy Spirit proceeds by an eternal procession from the Father and the Son. Yet, notwithstanding this difference as to origin, the Persons are co-eternal and co-equal: all alike are uncreated and omnipotent. This, the Church teaches, is the revelation regarding God's nature which Jesus Christ, the Son of God, came upon earth to deliver to the world: and which she proposes to man as the foundation of her whole dogmatic system.
In Scripture there is as yet no single term by which the Three Divine Persons are denoted together. The word trias (of which the Latin trinitas is a translation) is first found in Theophilus of Antioch about A.D. 180. He speaks of "the Trinity of God [the Father], His Word and His Wisdom (To Autolycus II.15). The term may, of course, have been in use before his time. Afterwards it appears in its Latin form of trinitas in Tertullian (On Pudicity 21). In the next century the word is in general use. It is found in many passages of Origen ("In Ps. xvii", 15). The first creed in which it appears is that of Origen's pupil, Gregory Thaumaturgus. In his Ekthesis tes pisteos composed between 260 and 270, he writes: .....continue reading at the link above.
The trinity is mentioned in the Didache.
Try this — 50 AD — while the Apostles were still alive.
http://www.catholic.com/video/what-is-the-didache