Where something plural resounds in the Godhead, not a duality is to be understood, but a Trinity, as is related in the very destruction of the city of Sodom. The Lord, it says, appeared to Abraham as he sat in the door of his tent, in the very heat of the day; and when he had lifted up his eyes, three men appeared to him; and when he had seen them, he ran to meet them, and bowed himself to the ground, and said: Lord, if I have found grace in your eyes, do not pass by your servant (Gen. 18:1-3). Surely in none of these three did either a more adorned dress or a more eminent form excel. And yet Abraham, understanding the mystery of the undivided Trinity, entreats the three by one name, because the person is threefold in unity, and the substance is one in the Trinity;
of which the Apostle also says: One Lord, one faith, one baptism (Eph. 4:5). Who would presume to confound, against the vessel of election, this unity with plurality? For he knew that the Holy Spirit reigns as Lord together with the Father and the Son, to whom he consecrated our bodies as a dwelling, saying: Do you not know that you are the temple of God, and the Spirit of God dwells in you (1 Cor. 3:16)? By what testimony shall the Holy Spirit ever be proved God more than by this, that we are His dwelling place, and God dwells in us? But Paul had long since owed such belief to the divine calling.
For indeed in the Acts of the Apostles it is written: As they were ministering and fasting, the Holy Spirit said, Set apart for me Paul and Barnabas for the work to which I have called them (Acts 13:2). This calling is not from the command of a superior, but spontaneous, as I judge. He commands them to be set apart for Himself; He says they are taken up for Himself. Without doubt He is God who inspires, chooses, and sends the apostles. And yet the same holy Paul, though at times he distinctly teaches that the Father, the Son, or the Holy Spirit is Lord, crowns the summit of the faith with that definition I mentioned above: One Lord, one faith, one baptism (Eph. 4:5). Thus therefore we can no more speak of two Gods than of two faiths or two baptisms.
Heathenism craves to name gods; the Jew believes that he is often washed from sins by the bath; truth forbids that faith be spoken of in the plural as though many, nor does the Latin tongue permit it. There is one Lord — we do not divide Him; one faith — we do not rend it; one baptism — we do not repeat it. Preserving its honor even in the reception of the Bonosiacs and others, if they answer that they were baptized in the name of the Father and of the Son and of the Holy Spirit, what they have rightly confessed we receive by keeping it; what they had wrongly believed we heal by blessing; and what had been lacking, after the naming has been set forth, is supplied by belief made firm.