Joh 1:14 And the Word was made flesh, and dwelt among us, (and we beheld his glory, the glory as of the only begotten of the Father,) full of grace and truth.
Joh 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.
Joh 3:16 For God so loved the world, that he gave his only begotten Son, that whosoever believeth in him should not perish, but have everlasting life.
Joh 3:18 He that believeth on him is not condemned: but he that believeth not is condemned already, because he hath not believed in the name of the only begotten Son of God.
It has been argued by Jehovah’s Witnesses (and Mormons) that the Bible teaches that Jesus is not divine, but an exalted creature. Appeal is made to the above verses and Hebrews 11:17 - By faith Abraham, when he was tried, offered up Isaac: and he that had received the promises offered up his only begotten son - in which Abraham “offered up Isaac”, he “offered up his only begotten son”. As in Isaac’s case, who was “only begotten” in the normal sense, so with Jesus, who “in the same sense that an earthly father, like Abraham, begets a son” – so say the Jehovah’s Witnesses.
The words “only begotten” in the verses above, do not mean that Christ was created, but that He is unique, specially blessed or favoured. B.B. Warfield wrote:
“The adjective ‘only begotten’ conveys the idea, not of derivation and subordination, but of uniqueness and consubstantiality: Jesus is all that God is and He alone is this”.
Consubstantial means, ‘of the same substance or essence’. (Christ is consubstantial with the Father).
Thinking about Abraham and Isaac, Isaac was not Abraham’s “only begotten” son in the sense that he was the only son Abraham had. Abraham had a number of sons, including Ishmael, whose birth preceded Isaac’s. Isaac was Abraham’s “only begotten” son in the sense that he was Abraham’s favoured, unique son. If God is Jesus’ Father “in the same sense that an earthly father begets a son,” then it would seem that Jesus must have had a heavenly Mother as well as a heavenly Father.
The Jehovah’s Witnesses agree that -
So, since the Father is eternal, His Son must also be eternal, because the term Father is meaningless apart from Son.
Jesus Christ was thus the only begotten Son before the incarnation.
R.C. Sproul, “Essential Truths of the Christian Faith”
Ron Rhodes, “Reasoning from the Scriptures with the Jehovah’s Witnesses”