5 But with Christ's bodily departure, both the Father and the Son, as well as the Holy Spirit, were spiritually present with them. For had Christ departed from them in such a sense that it would be in His place, and not along with Him, that the Holy Spirit would be present in them, what becomes of His promise when He said, “Lo, I am with you always, even to the end of the world;” and, I and the Father “will come unto him, and will make Our abode with him;” seeing that He also promised that He would send the Holy Spirit in such a way that He would be with them for ever?
In this way it was, on the other hand, that seeing they were yet out of their present carnal or animal condition to become spiritual, with undoubted certainty also were they yet to have in a more comprehensive way both the Father, and the Son, and the Holy Spirit. But in no one are we to believe that the Father is present without the Son and the Holy Spirit, or the Father and the Son without the Holy Spirit, or the Son without the Father and the Holy Spirit, or the Holy Spirit without the Father and the Son, or the Father and the Holy Spirit without the Son; but wherever any one of Them is, there also is the Trinity, one God.
But here the Trinity had to be suggested in such a way that, although there was no diversity of essence, yet the personal distinction of each one separately should be presented to notice; where those who have a right understanding can never imagine a separation of natures.
Source: Tractates on the Gospel of John (New Advent)