Abraham believed God

Says Galatians 3:6 reads Ἀβραὰμ ἐπίστευσεν τῷ θεῷ, καὶ ἐλογίσθη αὐτῷ εἰς δικαιοσύνην (Abraam episteusen tō theō kai elogisthē autō eis dikaiosynēn, “Abraham believed God, and it was reckoned to him as righteousness”). The BSB has “Abraham believed God, and it was credited to him as righteousness.” The wording is that of Genesis 15:6, “Abram believed the Lord, and it was credited to him as righteousness”; see Gen 15:6 → Gal 3:6 and Doctrinal note: God credits righteousness, and faith is the instrument (Romans 4:3–8). In 3:7 and 3:9 the phrase is οἱ ἐκ πίστεως (hoi ek pisteōs, “those of faith”), which the BSB renders “those who have faith”; 3:7 adds that these are sons of Abraham. In 3:8 the Scripture, foreseeing that ἐκ πίστεως δικαιοῖ τὰ ἔθνη ὁ θεὸς (ek pisteōs dikaioi ta ethnē ho theos, “God justifies the Gentiles by faith”), announced the gospel beforehand to Abraham: Ἐνευλογηθήσονται ἐν σοὶ πάντα τὰ ἔθνη (Eneulogēthēsontai en soi panta ta ethnē, “all the nations will be blessed in you”). Genesis 12:3 has “all the families of the earth will be blessed through you”; see Gen 12:3 → Gal 3:8. The wording is treated in Those of faith, and the sons of Abraham (3:6–9).

Implies Because Abraham was counted righteous by believing God, and because the promise that all nations would be blessed in him is read as God justifying the Gentiles by faith, those of faith are his sons.