9 Therefore, beloved, let us see what those men desire not to see; not what they may not see, but what they grieve to see, as though it were shut against them. Whither were the disciples sent to baptize as ministers, in the name of the Father, and of the Son, and of the Holy Ghost? Whither were they sent? “Go,” said He, “baptize the nations.” You have heard, brethren, how that inheritance comes, “Ask of me, and I will give You the nations for Your inheritance, and the utmost bounds of the earth for Your possessions.” You have heard how that “from Sion went forth the law, and the word of the Lord from Jerusalem.” For it was there the disciples were told, “Go, baptize the nations in the name of the Father, and of the Son, and of the Holy Ghost.” We became attentive when we heard, “Go, baptize the nations.”
In whose name? “In the name of the Father, and of the Son, and of the Holy Ghost.” This is one God; for it says not in the “names” of the Father, and of the Son, and of the Holy Ghost, but “in the name of the Father, and of the Son, and of the Holy Ghost.” Where you hear one name, there is one God; just as it was said of Abraham's seed, and the Apostle Paul expounds it, “In your seed shall all nations be blessed; he said not, In seeds, as in many, but as in one, and in your seed which is Christ.” Wherefore, just as the apostle wished to show you that, because in that place it is not said “in seeds,” Christ is one; so here too, when it is said, “in the name,” not in the names, even as these, “in seed,” not in seeds, is it proved that the Father, and the Son, and the Holy Ghost are one God.
Source: Tractates on the Gospel of John (New Advent)