John 1:18 — No man hath seen God at any time; the only begotten Son, which is in the bosom of the Father, He hath declared Him.
Old Testament manifestations of God were the Son.
The Son came to make the invisible visible.
And He said, Thou canst not see My face: for there shall no man see Me, and live (Exodus 33:20). Not that any man hath seen the Father, save He which is of God, He hath seen the Father (John 6:46).
only begotten Son — Literally "an only one, God"
is (not was) in the bosom — indicates closeness, comfort, companionship
declared Him = explained Him. No man can know God except through Jesus Christ, the expression of God (John 1:1).
In the Old Testament, Moses saw only the back of God — Law. And I will take away Mine hand, and thou shalt see My back parts: but My face shall not be seen (Exodus 33:23).
But we see the face of Christ — Grace. For God, who commanded the light to shine out of darkness, hath shined in our hearts, to give the light of the knowledge of the glory of God in the face of Jesus Christ (2 Corinthians 4:6).