"No man hath seen God at any time; the only begotten Son, which is in the bosom of the Father, He hath declared Him" (John 1:18).
God, in His essential being, is omnipresent. "Whither shall I go from thy Spirit? or whither shall I flee from thy presence?" (Psalm 139:7). This rhetorical question of David's has the obvious answer that one can never escape God's presence; He is present everywhere in His creation. Being present everywhere, He is necessarily invisible anywhere. He is "the King eternal, immortal, invisible, the only wise God" (I Timothy 1:17).
Yet, although God is omnipresent, He is also omnipotent, and He can therefore manifest Himself in tangible, visible form when He so chooses. This He has done at various times through the ages, as on the occasion when He, with two angels, appeared to Abraham in the form of three men (Genesis 18).
Our text resolves any apparent contradictions in these truths by noting that, when God manifests Himself visibly to man, He does so in the person of His only begotten Son. The Lord Jesus Christ, in fact, is "the image of the invisible God, the firstborn of every creature" (Colossians 1:15). In some marvelous way beyond human comprehension, the omnipresent, infinite God has on occasion taken on the appearance of human beings in order to convey a specific revelation or accomplish some divine purpose. Such manifestations are called theophanies and each has been implemented by the Son of God in pre-incarnate form.
Finally, however, the only begotten Son of God became also the eternal Son of man. "For the life was manifested, and we have seen it, and bear witness, and show unto you that eternal life, which was with the Father, and was manifested unto us" (I John 1:2). HMM