18 But godless unbelief finds it very hard to apprehend the true faith. Their capacity for devotion has never been expanded by belief, and is too narrow to receive a full presentment of the truth. Hence the unbelieving soul cannot grasp the great work done by God in being born as Man to accomplish the salvation of mankind; in the work of its salvation it fails to see the power of God. They think of the travail of His birth, the feebleness of infancy, the growth of childhood, the attainment of maturity, of bodily suffering and of the Cross with which it ended, and of the death upon the Cross; and all this conceals His true Godhead from their eyes.
Yet He had called into being all these capacities for Himself, as additions to His nature; capacities which in His true Divine nature He had not possessed. Thus He acquired them without loss of His true Divinity, and ceased not to be God when He became Man; when He, Who is God eternally, became Man at a point in time. They cannot see an exercise of the true God's power in His becoming what He was not before, yet never ceasing to be His former Self. And yet there would have been no acceptance of our feeble nature, had not He by the strength of His own omnipotent nature, while remaining what He was, come to be what previously He was not.
What blindness of heresy, what foolish wisdom of the world, which cannot see that the reproach of Christ is the power of God, the folly of faith the wisdom of God! So Christ in your eyes is not God because He, Who was from eternity, was born, because the Unchangeable grew with years, the Impassible suffered, the Living died, the Dead lives; because all His history contradicts the common course of nature! Is not all this simply to say that He, being God, was omnipotent? Not yet, you holy and venerable Gospels, do I turn your pages, to prove from them that Christ Jesus, amid these changes and sufferings, is God.
For the Law is the forerunner of the Gospels, and the Law must teach us that, when God clothed Himself in infirmity, He lost not His Godhead. The types of the Law are our convincing assurance of the mysteries of the Gospel faith.
Source: On the Trinity (New Advent)