44 To me, who hold that God cannot be known except by devotion, even to answer such objections seems no less unholy than to support them. What presumption to suppose that words can adequately describe His nature, when thought is often too deep for words, and His nature transcends even the conceptions of thought! What blasphemy even to discuss whether anything is lacking in God, whether He is Himself full, or it remains for Him to be fuller than His fullness! If God, Who is Himself the source of His own eternal divinity, were capable of progress, that He should be greater today than yesterday, He could never reach the time when nothing would be wanting to Him, for the nature to which advance is still possible must always in its progress leave some ground ahead still untrodden: if it be subject to the law of progress, though always progressing it must always be susceptible of further progress.
But to Him, Who abides in perfect fullness, Who for ever is, there is no fullness left by which He can be made more full, for perfect fullness cannot receive an accession of further fullness. And this is the attitude of thought in which reverence contemplates God, namely, that nothing is wanting to Him, that He is full.
Source: On the Trinity (New Advent)