The Greek word mo·no·ge·nesʹ is defined by lexicographers as “single of its kind, only,” or “the only member of a kin or kind.” (Thayer’s Greek-English Lexicon of the New Testament, 1889, p. 417; Liddell and Scott’s Greek-English Lexicon, Oxford, 1968, p. 1144) The term is used in describing the relation of both sons and daughters to their parents.
The Scriptures speak of “the only-begotten son” of a widow who lived in the city of Nain, of Jairus’ “only-begotten daughter,” and of a man’s “only-begotten” son whom Jesus cured of a demon. (Lu 7:11, 12; 8:41, 42; 9:38) The Greek Septuagint uses mo·no·ge·nesʹ when speaking of Jephthah’s daughter, concerning whom it is written: “Now she was absolutely the only child. Besides her he had neither son nor daughter.”
The apostle John repeatedly describes the Lord Jesus Christ as the only-begotten Son of God. (Joh 1:14; 3:16, 18; 1Jo 4:9) This is not in reference to his human birth or to him as just the man Jesus. As the Loʹgos, or Word, “this one was in the beginning with God,” even “before the world was.” (Joh 1:1, 2; 17:5, 24) At that time while in his prehuman state of existence, he is described as the “only-begotten Son” whom his Father sent “into the world.”
He is described as having “a glory such as belongs to an only-begotten son from a father,” the one residing “in the bosom position with the Father.” (Joh 1:14, 18) It is hard to think of a closer, more confidential, or more loving and tender relationship between a father and his son than this.
The angels of heaven are sons of God even as Adam was a “son of God.” (Ge 6:2; Job 1:6; 38:7; Lu 3:38) But the Loʹgos, later called Jesus, is “the only-begotten Son of God.” (Joh 3:18) He is the only one of his kind, the only one whom God himself created directly without the agency or cooperation of any creature. He is the only one whom God his Father used in bringing into existence all other creatures. He is the firstborn and chief one among all other angels (Col 1:15, 16; Heb 1:5, 6), which angels the Scriptures call “godlike ones” or “gods.” (Ps 8:4, 5) Therefore, according to some of the oldest and best manuscripts, the Lord Jesus Christ is properly described as “the only-begotten god [Gr., mo·no·ge·nesʹ the·osʹ].”
A few translations, in support of the Trinitarian “God the Son” concept, would invert the phrase mo·no·ge·nesʹ the·osʹ and render it as “God only begotten.” But W. J. Hickie in his Greek-English Lexicon to the New Testament (1956, p. 123) says it is hard to see why these translators render mo·no·ge·nesʹ hui·osʹ as “the only begotten Son,” but at the same time translate mo·no·ge·nesʹ the·osʹ as “God only begotten,” instead of “the only begotten God.”
Paul referred to Isaac as Abraham’s “only-begotten son” (Heb 11:17), even though Abraham also fathered Ishmael by Hagar as well as several sons by Keturah. (Ge 16:15; 25:1, 2; 1Ch 1:28, 32) God’s covenant, however, was established only through Isaac, Abraham’s only son by God’s promise, as well as the only son of Sarah. (Ge 17:16-19) Furthermore, at the time Abraham offered up Isaac, he was the only son in his father’s household. No sons had yet been born to Keturah, and Ishmael had been gone for some 20 years
So from several viewpoints in regard to the promise and the covenant, the things about which Paul was writing to the Hebrews, Isaac was Abraham’s only-begotten son. Hence, Paul parallels “the promises” and the “only-begotten son” with “‘your seed’ . . . through Isaac.” (Heb 11:17, 18) Whether Josephus had a similar viewpoint or not, he too spoke of Isaac as Abraham’s “only son.”