We proceed to the eighth article thus:
1. It seems that explicit belief in the Trinity has not been necessary for salvation. The apostle indeed says in Heb.11:6: "he that cometh to God must believe that he is, and that he is a rewarder of them that diligently seek him." But one can believe this without believing in the Trinity. Hence it has not been necessary to believe in the Trinity explicitly.
2. Again, in John 17:6 the Lord says: "I have manifested thy name unto the men which thou gavest me." Expounding this, Augustine says: "Not thy name whereby thou art called God, but thy name whereby thou art called my Father," and he adds later: "He is known among all nations as the God who made the world; he is known in Judea as the God who is not to be worshipped together with false Gods; but he has not manifested unto men this name which was formerly hidden from them, by which he is called the Father of this Christ through whom he taketh away the sin of the world" (Tract 106 in Joan.). Thus it was not known, before the coming of Christ, that both Fatherhood and Sonship were in the Godhead. Hence the Trinity was not believed explicitly.
3. Again, what we are required to believe explicitly about God is that the object of blessedness is in God. Now the object of blessedness is the supreme good, and we can understand that this is in God without distinguishing between the Persons. Hence it has not been necessary to believe in the Trinity explicitly.
On the other hand: the Trinity of the Persons is expressed in many ways in the Old Testament. It is said at the very beginning of Genesis, for example, in order to express the Trinity, "Let us make man in our image, after our likeness" (Gen.1:26). Explicit belief in the Trinity has therefore been necessary for salvation from the very beginning.
I answer: it is impossible to believe explicitly in the mystery of the incarnation of Christ without faith in the Trinity. For the mystery of the incarnation of Christ includes that the Son of God took flesh, that he made the world new through the grace of the Holy Spirit, and that he was conceived by the Holy Ghost. Hence just as before the time of Christ the mystery of his incarnation was believed explicitly by the wise, and implicitly and as it were obscurely by the simple, so also was the mystery of the Trinity believed in the same manner. But now that grace has been revealed, it is necessary for everybody to believe in the Trinity explicitly. Moreover, all who are born again in Christ are reborn through invocation of the Trinity, in accordance with Matt.28:19: "Go ye therefore, and teach all nations, baptizing them in the name of the Father, and of the Son, and of the Holy Ghost."
On the first point: to believe these two things has been necessary for all men at all times. But it is not sufficient for all men at all times.
On the second point: before the coming of Christ, faith in the Trinity was hidden in the faith of the wise. But it was made manifest to the world through Christ, and also through the apostles.
On the third point: without the Trinity of the Persons, the supreme goodness of God can be understood as we now understand it through its effects. But without the Trinity of the Persons it cannot be understood as it is in itself, and as it will be seen by the blessed. Moreover, it is the sending of the divine Persons that brings us to blessedness.