17 His merciful and mysterious self-revelations are in no wise inconsistent with His true heavenly nature; and His faithful saints never fail to penetrate the guise He has assumed in order that faith may see Him. The types of the Law foreshew the mysteries of the Gospel; they enable the Patriarch to see and to believe what hereafter the Apostle is to gaze on and publish. For, since the Law is the shadow of things to come, the shadow that was seen was a true outline of the reality which cast it.
God was seen and believed and worshipped as Man, Who was indeed to be born as Man in the fullness of time. He takes upon Him, to meet the Patriarch's eye, a semblance which foreshadows the future truth. In that old day God was only seen, not born, as Man; in due time He was born, as well as seen. Familiarity with the human appearance, which He took that men might behold Him, was to prepare them for the time when He should, in very truth, be born as Man. Then it was that the shadow took substance, the semblance reality, the vision life.
But God remained unchanged, whether He were seen in the appearance, or born in the reality, of manhood. The resemblance was perfect between Himself, after His birth, and Himself, as He had been seen in vision. As He was born, so He had appeared; as He had appeared, so was He born. But, since the time has not yet come for us to compare the Gospel account with that of the prophet Moses, let us pursue our chosen course through the pages of the Law. Hereafter we shall prove from the Gospels that it was the true Son of God Who was born as Man; for the present, we are showing from the Law that it was true God, the Son of God, Who appeared to the Patriarchs in human form.
For when One appeared to Abraham as Man, He was worshipped as God and proclaimed as Judge; and when the Lord rained from the Lord, beyond a doubt the Law tells us that the Lord rained from the Lord in order to reveal to us the Father and the Son. Nor can we for a moment suppose that when the Patriarch, with full knowledge, worshipped the Son as God, he was blind to the fact that it was true God Whom he worshipped.
Source: On the Trinity (New Advent)