What does it mean to live by the faith of the Son of God?
This expression describes the believer’s life as being sustained by the faith or faithfulness of the Son of God, who loved and gave Himself. Paul was crucified with Christ, yet he lived because Christ lived in him; God counts the old sin nature crucified when a person believes the gospel.
Les teaches from Galatians 2:20 that Paul was not literally put on a cross, but in the mind of God he was crucified with Christ. Paul was physically alive when he wrote it, yet he had been crucified. He says, “I am crucified with Christ: nevertheless I live,” and then, “yet not I, but Christ liveth in me.” The life Paul now lived in the flesh, he lived by the faith or faithfulness of the Son of God, who loved him and gave Himself for him. Les then ties this to Romans: as soon as we believe we are sinners, know the gospel is the only remedy, and believe it, God takes the old sin nature and considers it crucified. So living by the faith or faithfulness of the Son of God is living with Christ indwelling and the old sin nature reckoned crucified.
I am crucified with Christ: nevertheless I live; yet not I, but Christ liveth in me: and the life which I now live in the flesh I live by the faith of the Son of God, who loved me, and gave himself for me.