Compare John 1:18 in diffeent translations. First compare the KJV with the ESV translation.

“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, KJV

“No one has ever seen God; God the only Son, who is at the Father’s side, he has made him known.” John 1:18, ESV

Now compare the same verse in the NASB95 with the modern NASB.

“No one has seen God at any time; the only begotten God who is in the bosom of the Father, He has explained Him.” John 1:18 NASB 95

“No one has seen God at any time; God the only Son, who is in the arms of the Father, He has explained Him. John 1:18, NASB

The phrase “only begotten” translates the Greek word monogenes. This word is correctly variously translated into English as “only,” “one and only,” and “only begotten.” False teachers use this phrase to try to prove their assertion that Jesus Christ isn’t fully God; i.e., that he is a created being. They insist the word “begotten” means Jesus is a created being because only someone who had a beginning in time can be “begotten.” What this fails to note is that “begotten” is an English translation of a Greek word. We must consider the original meaning of the Greek word.

According to the Greek-English Lexicon of the New Testament and Other Early Christian Literature (BDAG, 3rd Edition), monogenes has two primary definitions:

The first definition is “pertaining to being the only one of its kind within a specific relationship.” This is its meaning in Hebrews 11:17 when the writer refers to Isaac as Abraham’s “only begotten son” (KJV). Abraham had more than one son, but Isaac was the only son he had by Sarah and the only son of the covenant. Therefore, it is the uniqueness of Isaac among the other sons that allows for the use of monogenes in that context.

The second definition is “pertaining to being the only one of its kind or class, unique in kind.” This is the meaning that is implied in John 3:16 (see also John 1:14, 18; 3:18; 1 John 4:9). John was primarily concerned with demonstrating that Jesus is the Son of God (John 20:31), and he uses monogenes to highlight Jesus as uniquely God’s Son—sharing the same divine nature as God—as opposed to believers who are God’s sons and daughters by adoption (Ephesians 1:5). Jesus is God’s “one and only” Son.

“Begotten” means an eternal, unique relationship. Thus, modern English translations use “one and only.” This “begetting” of the Son clearly is very unique because the Son had no beginning. Begotten means an eternal generation of the Son from the Father. Modern english translations no longer use the word “begotten” because people tend to misunderstand what the word means as the English language evolves.

Translations necessarily change as word meanings change. For example, the word “gay” used a hundred years ago meant merry, cheerful, bright, or showy. Today it means something entirely different – homosexual. The statement that “John is a gay person”meant something entirely different a hundred years ago than it does today.

“Begotten” really means an eternal, unique relationship.