In Galatians 3:26, the Apostle Paul writes: "For in Christ Jesus you are all sons of God, through faith." This verse clearly states that Christians become children of God through their faith in Christ Jesus, not through adherence to the Mosaic law or any other means.
This teaching is central to Paul's argument in Galatians, where he contends against the Judaizers who insisted that Gentile converts must observe the Jewish law (particularly circumcision) to be fully accepted by God. Paul argues that the law served as a guardian or tutor until Christ came (Galatians 3:24-25), but now that faith has come, believers are no longer under that guardian. Sonship comes through faith, not through law-keeping.
Paul continues in Galatians 3:27-29 by explaining that all who have been baptised into Christ have "put on" Christ, and in Him there is neither Jew nor Gentile, slave nor free, male nor female - all are one in Christ Jesus and heirs according to the promise made to Abraham.
The other options are incorrect: subjecting oneself to the rule of law is the very approach Paul refutes; being an heir of Abraham is a consequence of faith, not its means; and confessing Jesus at fellowship is not a concept from this passage.