In some places, the Bible uses wording like "children of God" or "sons of God". One example of this use is Genesis 6:2, and another is Galatians 3:26:
26 For through faith you are all children of God in Christ Jesus.
26 You are all sons of God through faith in Christ Jesus, [alternate translation]
I understand that many of these uses might be symbolic. Does the Bible clarify?
According to the Bible and Christian teaching, was Jesus the only Son of God?
The Greek word /monogenes/ is the key here. I did a word study on this a number of years back. The word actually appears in the Septuagint (the Greek translation of the Hebrews Scriptures). Here it is actually used in reference to Isaac as the /monogenes/ son of Abraham.
Interestingly enough, Isaac was not the only son of Abraham and not even the first. Ishmael was born first and Abraham had six other children with Keturah after the death of Sarah.
So, /monogenes/ does not indicate that Isaac was the only son. The word comes from the combination of two other words -- /mono/ and /genes/. /mono/ is, of course, indicative of "one". /genes/ refers to kind. In a sense, then, /monogenes/ refers to "one of a kind" or "something wholly unlike any other".
Since Isaac was not the only son of Abraham, in what sense was he a one of a kind? Specifically, he was unique in his miraculous birth and in his being the fulfillment of the promise of God. No other son was born in fulfillment of a promise from God and no other son was born miraculously.
Jesus is identified as the /monogenes/ Son of God. Typically, this is translated as "Only Begotten". It is true that Jesus is not the only son of God or only child of God, because, as you identified, all of those who have faith in Jesus are children of God.
However, the Sonship of Jesus is unique. Indeed, His Sonship is wholly unlike any other. Its uniqueness is that it is eternal. Jesus never became a son of God, but was always the Son of God and God the Son.
So, Jesus is not the only son of God, but He is a Son unlike any other in His divinity.
Jesus is not God the Father's only son, but he is God the Father's only-begotten Son. And, μονογενής means "only-born" (or "only-begotten") not "one of a kind."
The the suffix —γενής is found in many Greek words, including εὐηγενής ("well born"), ἀγενής ("low born"), ἀλλογενής ("other born"), πατρογενής ("father born"), πυριγενής/ πυρογενής ("fire born"), γηγενής ("earth born") and θεογενής ("God born").
Jesus is the only Son of God. However, Christians are the children of God via adoption. See Romans 8:15, Galatians 4:5, and Ephesians 1:5. Note that Angels are also referred to as the sons of God (see Job 1:6; Gen 6:2 [there is some debate whether Gen 6:2 refers to angels or men]). However, the reference to angels is figurative.
μονογενής is the subject of some etymological debate. It comes from mono meaning "one" and either ginomai meaning become or geneo meaning beget. Current scholarly opinion vastly favors "become." Thus μονογενής means "special in kind."
We say Jesus is the only Son of God not because of μονογενής but because he is the Firstborn of the Father and there are no other member of the Trinity than Father, Son, and Spirit.
