A.T. Robertson Commentary John 1:18

"No man hath seen God at any time; the only begotten Son, who is in the bosom of the Father, he hath declared [him]." — John 1:18 (ASV)

No man hath seen God at any time (θεον ουδεις εωρακεν πωποτε). "God no one has ever seen." Perfect active indicative of οραω. Seen with the human physical eye, John means. God is invisible (Exodus 33:20; Deuteronomy 4:12). Paul calls God αορατος (Colossians 1:15; 1 Timothy 1:17). John repeats the idea in Joh 5:37; 6:46. And yet in 14:7 Jesus claims that the one who sees him has seen the Father as here.

The only begotten Son (ο μονογενης υιος). This is the reading of the Textus Receptus and is intelligible after ως μονογενους παρα πατρος in verse 14. But the best old Greek manuscripts (Aleph B C L) read μονογενης θεος (God only begotten) which is undoubtedly the true text. Probably some scribe changed it to ο μονογενης υιος to obviate the blunt statement of the deity of Christ and to make it like 3:16. But there is an inner harmony in the reading of the old uncials. The Logos is plainly called θεος in verse 1. The Incarnation is stated in verse 14, where he is also termed μονογενης. He was that before the Incarnation. So he is "God only begotten," "the Eternal Generation of the Son" of Origen's phrase.

Which is in the bosom of the Father (ο ων εις τον κολπον του πατρος). The eternal relation of the Son with the Father like προς τον θεον in verse 1. In 3:13 there is some evidence for ο ων εν τω ουρανω used by Christ of himself while still on earth. The mystic sense here is that the Son is qualified to reveal the Father as Logos (both the Father in Idea and Expression) by reason of the continual fellowship with the Father.

He (εκινος). Emphatic pronoun referring to the Son.

Hath declared him (εξηγησατο). First aorist (effective) middle indicative of εξηγεομα, old verb to lead out, to draw out in narrative, to recount. Here only in John, though once in Luke's Gospel (24:35) and four times in Ac (10:8; 15:12,14; 21:19). This word fitly closes the Prologue in which the Logos is pictured in marvellous fashion as the Word of God in human flesh, the Son of God with the Glory of God in him, showing men who God is and what he is.