“Therefore know that only those who are of faith are sons of Abraham”

Therefore

Paul draws a conclusion from the quotation from Genesis 15:6 in the previous verse. Paul whacks a great wallop against the Judaizers by relating Abraham’s salvation to the principle of grace. The legalists taught that a person became a Jew by circumcision, but Abraham became a Jew by faith. God declared him right as He was right by faith. 

know that only those who are of faith

God justified Abraham by faith, and he justifies us by faith. Faith in the finished work of Christ is the starting point of being a Christian. We can only know this from the experience of personal faith in Christ.

God bases our relationship with Himself on the foundation of spiritual birth, not natural birth. He saves us through spiritual regeneration, not natural generation.

are sons of Abraham

Only those who exercise faith in God’s promises have the right to identify with Abraham. We follow Abraham’s principle of salvation by grace through faith when we accept God’s grace.

The Judaizers reversed the sequence of salvation and circumcision. They taught that circumcision was a prerequisite to salvation (Romans 2:28-29). Physical relation to Abraham is not the heart of salvation. Abraham became a Jew by faith; he was circumcised after his salvation. All those who follow his faith are his spiritual sons. Faith, not family lineage, establishes a person as a true believer.

Note that Paul uses the word “sons,” not “children” here. The idea of “sons” in Greek is that those who exercise faith carry official privilege and position with God.

As Abraham exercised faith in Jesus Christ (Jehovah of the Old Testament), he became an adult son with adult spiritual privileges. In Jewish tradition, a son could not marry, enter the army or do business on his own until he turned 13. On his thirteenth birthday, his father gave him a toga of adulthood.

Principle:

Faith is the sole means of salvation, whether in the Old or New Testament.

Application:

Genuine born-again people place mutually exclusive trust in the person and work of Christ for their salvation. Whether they place their hope in the coming of the Messiah in the Old Testament or Christ in the New Testament, it is always by faith.

Do you take God at His naked unsupported Word? God will regenerate you by faith, not because you come from a fine family.