21 Thus, after the present first book, the second expounds the mystery of the Divine birth, that those who shall be baptized in the Name of the Father and of the Son and of the Holy Ghost may know the true Names, and not be perplexed about their sense but accurately informed as to fact and meaning, and so receive full assurance that in the words which are used they have the true Names, and that those Names involve the truth.
Source: On the Trinity (New Advent)