He emptied Himself not in such a way that He was void of His own fulness, but in order that He, whose fulness I could not endure, might infuse Himself into me according to the measure of my capacity.
In like manner the Father says that He pours out of the Spirit upon all flesh; for He did not pour Him forth wholly, but that which He poured forth abounded for all.
[...] God shed forth what He deemed to be sufficient for us, and what was shed forth is not separated nor divided, but has a unity of fulness whereby He may enlighten the sight of our hearts according to our capacity to receive.
We receive so much as the advancing of our mind acquires, for the fulness of the grace of the Spirit is indivisible, and we share in this grace according to the capacity of our own nature.
God, then, sheds forth of the Spirit, and the love of God is also shed abroad through the Spirit. As God shed forth of the Holy Spirit, so also “the love of God is shed abroad in our hearts through the Holy Spirit” (Romans 5:5) in order that we may understand that the Holy Spirit is not a work, but is the dispenser and plenteous fount of the divine love.
[...] Wisdom, which proceeds from the mouth of God, cannot be said to be created, nor the Word Which is uttered from His heart, nor the power in which is the fulness of the eternal Majesty.
So, too, the Spirit which is poured forth from the mouth of God cannot be considered to be created, since God Himself has shown their unity to be such that He speaks of His pouring forth of His Spirit.
By which we understand that the grace of God the Father is the same as that of the Holy Spirit, and that, without any division or loss, it is divided to the hearts of each.
Ambrose of Milan (c. 337-397): On the Holy Spirit, 1,8,92-97.