The idea of a human or group of humans being God’s son is not uncommon in the Hebrew Tanakh (“Old Testament”). For example, in Exo. 4:22,1 it is written,
22 Thus said Yahveh, “Israel is My son, even My firstborn.”
The motif of the nation of Israel being God’s child is reiterated in various other books of the Tanakh. In Deu. 32:6, Speaking of Yahveh, Moses asks the Israelites,
6 “Do you thus requite Yahveh, O’ foolish and unwise people? Is He not your father who bought you? Has he not made you and established you?”
Similarly, in Mal. 2:10, it is written,
10 Have we not all one father? Has not one God created us? Why do we deal treacherously every man against his brother, by profaning the covenant of our fathers?
But, of course, none of the Israelites were ever actually begotten by God the Father. They weren’t literally God’s sons or daughters. Rather, they were sons and daughters by adoption.2 They were called sons and daughters, but all had actual human parents of whom they were begotten as sons and daughters.
Accordingly, some might insist that the Lord Jesus Christ, despite being called the “son of God,” is no different than any other Israelite being called God’s son. However, we should note that he distinguishes his relationship with the Father to that of everyone else’s (i.e., other Israelites) relationship to the Father.
For example, in John 20:17, it is written,
17 Jesus says to her, “Do not touch me, for I have not yet ascended to my Father, but go to my brothers and say to them, ‘I ascend to my Father and your Father, to my God and your God.’”
Why didn’t he simply say, “I ascend to our Father and to our God”? Instead, he explicitly distinguishes his relationship with the Father to theirs. There’s also no doubt that his words were understood as something more than the historical status quo. His statement that God was his Father was understood as blasphemous, worthy of death.
For example, in John 5:17–18, it is written,
17 But Jesus answered them, “My Father works until now, and I work.” 18 Therefore, the Jews sought to kill him even more, because he not only had broken the Sabbath, but he also said that God was his Father, making himself equal with God.
He does not say, “Our Father.” Therefore, in one way [He is] mine; in another way, [He is] yours. By nature [He is] mine, by grace [He is] yours. “And my God and your God.” Nor did he say here, “Our God.” Therefore, here also, in one way [He is] mine; in another way, [He is] yours. [He is] my God under whom I am man; [He is] your God, between whom and Himself I am mediator.
Non ait: Patrem nostrum: aliter ergo meum, aliter vestrum; natura meum, gratia vestrum. Et Deum meum, et Deum vestrum. Neque hic dixit: Deum nostrum: ergo et hic aliter meum, aliter vestrum; Deum meum sub quo et ego homo sum, Deum vestrum inter quos et ipsum mediator sum.
So, his identity as the “son of God” is indeed different from Christians as well as the Israelites before. He is the son of God by nature, both physically and spiritually begotten by God the Father. On the other hand, Christians are spiritually begotten by the Holy Spirit via regeneration (being born again), but they are physically begotten by their human parents.
John of Damascus wrote,4
Wherefore he said, “I ascend to my Father and your Father.” He did not say “our Father,” but “my Father,” clearly [in the sense of Father] by nature (φύσει), and “your Father,” [in the sense of Father] by grace (χάριτι).
Ὅθεν ἔλεγεν· «Ἀναβαίνω πρὸς τὸν πατέρα μου καὶ πατέρα ὑμῶν.» Οὐκ εἶπε· πατέρα ἡμῶν, ἀλλὰ «πατέρα μου», φύσει δῆλον, καὶ «πατέρα ὑμῶν» χάριτι.
In summary, the reason why his admission of being the “son of God” was considered blasphemy by the chief priest was because the chief priest acknowledged it (just as others did in the Gospel of John) as a claim that the Lord Jesus Christ was likewise God in nature, and of course, the chief priest didn’t believe that claim was true. I’m not exactly sure it was his latter claim about coming upon the clouds of heaven that was the impetus for the chief priest’s accusation of the Lord Jesus Christ committing blasphemy. In my opinion, it’s likely the former admission of him admitting (or rather, not denying) that he is the son of God. It’s unfortunate that even many Christians neglect to recognize the import of individuals claiming the Lord Jesus Christ (and he himself admitting) to be the son of God.
1 Hos. 11:1
2 Greek ἡ υἱοθεσία; cf. Rom. 9:4
3 John 20:17, Tractate 121 (CXXI), Chapter 20 (XX), §3, pp. 1957–1958
4 Book 4 (IV), Ch. 8 (Ηʹ), p. 1117
Aurelius Augustinus Hipponensis. Patrologiæ Cursus Completus: Series Prima. “In Joannis Evangelium Tractatus CXXIV” (“124 Tractates on the Gospel of John”). Ed. Migne, Jacques Paul. Vol. 35. Petit-Montrouge: Imprimerie Catholique, 1845.
John of Damascus (Ἰωάννης ὁ Δαμασκηνός). Patrologiæ Cursus Completus: Series Græca Prior. “ΕΚΔΟΣΙΣ ΑΚΡΙΒΗΣ ΤΗΣ ΟΡΘΟΔΟΞΟΥ ΠΙΣΤΕΩΣ” (“Accurate Exposition of the Orthodox Faith”). Ed. Migne, Jacques Paul. Vol. 94. Petit-Montrouge: Imprimerie Catholique, 1864.
This answer originally written by Der Übermensch under previous account.