And though some one there may be of so averse a spirit as to undertake to baptize in such sort as to omit one of these names, therein contradicting Christ Who ordained this for a law, his baptism will effect nothing; those who are baptized by him will not be at all delivered from their sins. From these words we gather how undivided is the substance of the Trinity, that the Father is verily the Father of the Son, and the Son verily the Son of the Father, and the Holy Spirit the Spirit of both the Father and the Son, and also the Spirit of wisdom and of truth, that is, of the Son of God. This then is the salvation of them that believe, and in this Trinity is wrought the perfect communication of ecclesiastical discipline.
Reproduced in full. Translation: Oxford 1841 (Newman), public domain (Public domain).
Also cites: Didymi Lib. ii, de Spir. Sanct.