John 1:18
Observe, therefore, with what fullness the Evangelist speaks; for having said that “no man has seen God at any time,” he does not go on to say, “that the Son who has seen, has declared Him,” but adds something beyond “seeing” by the words, “Who is in the bosom of the Father”; because, “to dwell in the bosom” is far more than “to see.” For he that merely “sees” has not an in every way exact knowledge of the object, but he that “dwells in the bosom” can be ignorant of nothing. Now lest when you hear that “none knows the Father, save the Son,” you should assert that although He knows the Father more than all, yet He knows not how great He is, the Evangelist says that He dwells in the bosom of the Father; and Christ Himself declares, that He knows Him as much as the Father knows the Son. Ask therefore the gainsayer, “Tell me, does the Father know the Son?” And if he be not mad, he will certainly answer “Yes.” Then ask again; “Does He see and know Him with exact vision and knowledge? Does He know clearly what He Is?” He will certainly confess this also. From this next collect the exact comprehension the Son has of the Father. For He says, “As the Father knows me, even so know I the Father”; and in another place, “Not that any man has seen the Father, save He which is of God.” Wherefore, as I said, the Evangelist mentions “the bosom,” to show all this to us by that one word; that great is the affinity and nearness of the Essence, that the knowledge is nowise different, that the power is equal. For the Father would not have in His bosom one of another essence, nor would He have dared, had He been one among many servants, to live in the bosom of his Lord, for this belongs only to a true Son, to one who has much confidence towards His Father, and who is in nothing inferior to Him.
Would you learn also His eternity? Hear what Moses says concerning the Father. When he asked what he was commanded to answer should the Jews enquire of him, “Who it was that had sent him,” he heard these words: “Say, I AM has sent me.” Now the expression “I AM,” is significative of Being ever, and Being without beginning, of Being really and absolutely. And this also the expression, “Was in the beginning,” declares, being indicative of Being ever; so that John uses this word to show that the Son Is from everlasting to everlasting in the bosom of the Father. For that you may not from the sameness of name, suppose that He is some one of those who are made sons by grace, first, the article is added, distinguishing Him from those by grace. But if this does not content you, if you still look earthwards, hear a name more absolute than this, “Only-Begotten.” If even after this you still look below, “I will not refuse,” says he, (St. John,) “to apply to God a term belonging to man, I mean the word 'bosom,' only suspect nothing degrading.” Do you see the lovingkindness and carefulness of the Lord? God applies to Himself unworthy expressions, that even so you may see through them, and have some great and lofty thought of Him; and do you tarry below? For tell me, wherefore is that gross and carnal word “bosom” employed in this place? Is it that we may suppose God to be a body? Away, he by no means says so. Why then is it spoken? For if by it neither the genuineness of the Son is established, nor that God is not a body, the word, because it serves no purpose, is superfluously thrown in. Why then is it spoken? For I shall not desist from asking you this question. Is it not very plain, that it is for no other reason but that by it we might understand the genuineness of the Only-Begotten, and His Co-eternity with the Father?
3. “He has declared Him,” says John. What has he declared? That “no man has seen God at any time”? That “God is one”? But this all the other prophets testify, and Moses continually exclaims, “The Lord your God is one Lord”; and Esaias, “Before Me there was no God formed, neither shall there be after me.” What more then have we learned from “the Son which is in the bosom of the Father”? What from “the Only-Begotten”? In the first place, these very words were uttered by His working; in the next place, we have received a teaching that is far clearer, and learned that “God is a spirit, and they that worship Him must worship Him in spirit and in truth”; and again, that it is impossible to see God; “that no man knows” Him, “save the Son”; that He is the Father of the true and Only-Begotten; and all other things that are told us of Him. But the word “has declared” shows the plainer and clearer teaching which He gave not to the Jews only but to all the world, and established. To the prophets not even all the Jews gave heed, but to the Only-Begotten Son of God all the world yielded and obeyed. So the “declaration” in this place shows the greater clearness of His teaching, and therefore also He is called “Word,” and “Angel of great Counsel.”
Since then we have been vouchsafed a larger and more perfect teaching, God having no longer spoken by the prophets, but “having in these last days spoken to us by His Son”, let us show forth a conversation far higher than theirs, and suitable to the honor bestowed on us. Strange would it be that He should have so far lowered Himself, as to choose to speak to us no longer by His servants, but by His own mouth, and yet we should show forth nothing more than those of old. They had Moses for their teacher, we, Moses' Lord. Let us then exhibit a heavenly wisdom worthy of this honor, and let us have nothing to do with earth. It was for this that He brought His teaching from heaven above, that He might remove our thoughts there, that we might be imitators of our Teacher according to our power. But how may we become imitators of Christ? By acting in everything for the common good, and not merely seeking our own. “For even Christ,” says Paul, “pleased not Himself, but as it is written, The reproaches of them that reproached You fell on Me.” Let no one therefore seek his own. In truth, a man (really) seeks his own good when he looks to that of his neighbor. What is their good is ours; we are one body, and parts and limbs one of another. Let us not then be as though we were rent asunder. Let no one say, “such a person is no friend of mine, nor relation, nor neighbor, I have nought to do with him, how shall I approach, how address him?” Though he be neither relation nor friend, yet he is a man, who shares the same nature with you, owns the same Lord, is your fellow-servant, and fellow-sojourner, for he is born in the same world. And if besides he partakes of the same faith, behold he has also become a member of you: for what friendship could work such union, as the relationship of faith? And our intimacy one with another must not be such nearness only as friends ought to show to friends, but such as is between limb and limb, because no man can possibly discover any intimacy greater than this sort of friendship and fellowship. As then you cannot say, “Whence arises my intimacy and connection with this limb?” (that would be ridiculous;) so neither can you say so in the case of your brother. “We are all baptized into one body”, says Paul. “Wherefore into one body?” That we be not rent asunder, but preserve the just proportions of that one body by our intercourse and friendship one with another.
Source: Homilies on the Gospel of John (New Advent)