17 And lest, perhaps, any one should say, And did not grace and truth come through Moses, who saw God, immediately he adds, “No one has seen God at any time.” And how did God become known to Moses? Because the Lord revealed Himself to His servant. What Lord? The same Christ, who sent the law beforehand by His servant, that He might Himself come with grace and truth. “For no one has seen God at any time.” And whence did He appear to that servant as far as he was able to receive Him?
But “the Only-begotten,” he says, “who is in the bosom of the Father, He has declared Him.” What signifies “in the bosom of the Father?” In the secret of the Father. For God has not a bosom, as we have, in our garments, nor is He to be thought of sitting, as we do, nor is He girt with a girdle so as to have a bosom; but because our bosom is within, the secret of the Father is called the bosom of the Father. And He who knew the Father, being in the secret of the Father, He declared Him.
“For no man has seen God at any time.” He then came and narrated whatever He saw. What did Moses see? Moses saw a cloud, he saw an angel, he saw a fire. All that is the creature: it bore the type of its Lord, but did not manifest the presence of the Lord Himself. For you have it plainly stated in the law: “And Moses spoke with the Lord face to face, as a friend with his friend.” Following the same scripture, you find Moses saying: “If I have found grace in Your sight, show me Yourself plainly, that I may see You.”
And it is little that he said this: he received the reply, “You can not see my face.” An angel then spoke with Moses, my brethren, bearing the type of the Lord; and all those things which were done by the angel promised that future grace and truth. Those who examine the law well know this; and when we have opportunity to speak somewhat of this matter also, we shall not fail to speak to you, beloved brethren, as far as the Lord may reveal to us.
Source: Tractates on the Gospel of John (New Advent)