41 I appeal not now to any of the titles which are given to the Son; there is no loss in delay when it is the result of an embarrassing abundance of choice. My present argument is that a successful result implies a sufficient cause; some clear and cogent motive must underlie every effectual performance. And so the Evangelist has been obliged to reveal his motive in writing. Let us see what is the purpose which he confesses;— But these things are written that you may believe that Jesus is the Christ, the Son of God. The one reason which he alleges for writing his Gospel is that all may believe that Jesus is the Christ, the Son of God.
If it be sufficient for salvation to believe that He is the Christ, why does he add The Son of God? But if the true faith be nothing less than the belief that Christ is not merely Christ, but Christ the Son of God, then assuredly the name of Son is not attached to Christ as a customary appendage due to adoption, seeing that it is essential to salvation. If then salvation consists in the confession of the name, must not the name express the truth? If the name express the truth, by what authority can He be called a creature? It is not the confession of a creature, but the confession of the Son, which shall give us salvation.
Source: On the Trinity (New Advent)