for “the Word was with God” (John 1:1), and not only with God but also in God;
for He says: “I am in the Father and the Father is in Me” (John 14:10).
[…] The Holy Spirit also…is not separated from the Father nor separated from the Son.
For how could He be separated from the Father Who is the Spirit of His mouth?
[…] He exists then, and abides always, Who is the Spirit of His mouth, but He seems to come down when we receive Him, that He may dwell in us, that we may not be alien from His grace.
To us He seems to come down, not that He does come down, but that our mind ascends to Him.
[…] The Spirit, then, so comes as does the Father, for where the Father is there is also the Son, and where the Son is there is the Holy Spirit.
The Holy Spirit, therefore, is not to be supposed to come separately.
But He comes not from place to place, but from the disposition of the order to the safety of redemption, from the grace of giving life to that of sanctification, to translate us from earth to heaven, from wretchedness to glory, from slavery to a kingdom.
The Spirit comes, then, as the Father comes. For the Son said, “I and the Father will come, and will make Our abode with Him” (John 14:23).
[…] Thus, then, comes the Spirit in Whom, when He comes, is the full presence of the Father and the Son.
But who can separate the Spirit from the Father and the Son, since we cannot even name the Father and the Son without the Spirit? “For no one says ‘Lord Jesus’, except in the Holy Spirit?” (1 Cor. 12:3).
If, then, we cannot call Jesus Lord except in the Holy Spirit, we certainly cannot proclaim Him without the Spirit.
But if the Angels also proclaim Jesus to be Lord, Whom no one can proclaim except in the Spirit, then in them also the office of the Holy Spirit operates.
[…] The presence and the grace of the Father, the Son, and the Holy Spirit are one, which is so heavenly and divine that the Son gives thanks therefore to the Father, saying,
“I give thanks to Thee, O Father, Lord of heaven and earth, because Thou hast hidden these things from the wise and prudent, and hast revealed them unto babes” (Matt. 11:25).
Ambrose of Milan (c. 337-397): On the Holy Spirit, Book 1, 11,120-125.