Objection 1. It would seem that it was not necessary for salvation to believe explicitly in the Trinity. For the Apostle says (Hebrews 11:6): "He that cometh to God must believe that He is, and is a rewarder to them that seek Him." Now one can believe this without believing in the Trinity. Therefore it was not necessary to believe explicitly in the Trinity.
Objection 2. Further our Lord said (John 17:5-6): "Father, I have manifested Thy name to men," which words Augustine expounds (Tract. cvi) as follows: "Not the name by which Thou art called God, but the name whereby Thou art called My Father," and further on he adds: "In that He made this world, God is known to all nations; in that He is not to be worshipped together with false gods, 'God is known in Judea'; but, in that He is the Father of this Christ, through Whom He takes away the sin of the world, He now makes known to men this name of His, which hitherto they knew not." Therefore before the coming of Christ it was not known that Paternity and Filiation were in the Godhead: and so the Trinity was not believed explicitly.
Objection 3. Further, that which we are bound to believe explicitly of God is the object of heavenly happiness. Now the object of heavenly happiness is the sovereign good, which can be understood to be in God, without any distinction of Persons. Therefore it was not necessary to believe explicitly in the Trinity.
On the contrary, In the Old Testament the Trinity of Persons is expressed in many ways; thus at the very outset of Genesis it is written in manifestation of the Trinity: "Let us make man to Our image and likeness" (Genesis 1:26). Therefore from the very beginning it was necessary for salvation to believe in the Trinity.
I answer that, It is impossible to believe explicitly in the mystery of Christ, without faith in the Trinity, since the mystery of Christ includes that the Son of God took flesh; that He renewed the world through the grace of the Holy Ghost; and again, that He was conceived by the Holy Ghost. Wherefore just as, before Christ, the mystery of Christ was believed explicitly by the learned, but implicitly and under a veil, so to speak, by the simple, so too was it with the mystery of the Trinity. And consequently, when once grace had been revealed, all were bound to explicit faith in the mystery of the Trinity: and all who are born again in Christ, have this bestowed on them by the invocation of the Trinity, according to Matthew 28:19: "Going therefore teach ye all nations, baptizing them in the name of the Father, and of the Son and of the Holy Ghost."
Reply to Objection 1. Explicit faith in those two things was necessary at all times and for all people: but it was not sufficient at all times and for all people.
Reply to Objection 2. Before Christ's coming, faith in the Trinity lay hidden in the faith of the learned, but through Christ and the apostles it was shown to the world.
Reply to Objection 3. God's sovereign goodness as we understand it now through its effects, can be understood without the Trinity of Persons: but as understood in itself, and as seen by the Blessed, it cannot be understood without the Trinity of Persons. Moreover the mission of the Divine Persons brings us to heavenly happiness.
Source: Summa Theologica (New Advent)