5 Take an illustration from the Holy Scriptures, whereby you may in some measure comprehend what I am saying. After our Lord Jesus Christ rose again, and was pleased to ascend into heaven, at the end of ten days He sent from thence the Holy Spirit, by whom those who were present in that one chamber were filled, and began to speak in the languages of all nations. The Lord's murderers, terrified by the miracle, were pricked to the heart and sorrowed; sorrowing, were changed; and being changed, believed.
There were added to the Lord's body, that is, to the number of believers, three thousand people. And so also by the working of another miracle there were added other five thousand. A considerable community was created, in which all, receiving the Holy Spirit, by whom spiritual love was kindled, were by their very love and fervor of spirit welded into one, and began in the very unity of fellowship to sell all that they had, and to lay the price at the apostles' feet, that distribution might be made to every one as each had need.
And the Scripture says this of them, that “they were of one soul and one heart toward God.” Give heed then, brethren, and from this acknowledge the mystery of the Trinity, how it is we say, There is both the Father, and the Son, and the Holy Spirit, and yet there is one God. See! There were so many thousands of these, and yet there was one heart; there were so many thousands, and one soul. But where? In God. How much more so God Himself? Do I err at all in word when I call two men two souls, or three men three souls, or many men many souls?
Surely I speak correctly. Let them approach God, and one soul belongs to all. If by approaching God many souls by love become one soul, and many hearts one heart, what of the very fountain of love in the Father and Son? Is it not still more so here that the Trinity is one God? For thence, of that Holy Spirit, does love come to us, as the apostle says: “The love of God is shed abroad in our hearts by the Holy Ghost, which is given unto us.” If then the love of God, shed abroad in our hearts by the Holy Ghost which is given unto us, makes many souls one soul, and many hearts one heart, how much rather are the Father and Son and Holy Spirit, one God, one light, and one beginning?
Source: Tractates on the Gospel of John (New Advent)