Why did God give Jacob a new name after he became a new man?
Because Jacob's wrestling demonstrated persistent faith, and God used that faith to transform him. This was his true salvation and new birth, making him a new man in God and in Christ. Since he was no longer the natural deceiver, God gave him a new name, Israel, meaning prince with God.
Les teaches that God saw faith in Jacob when Jacob wrestled with the Lord and refused to let go until He blessed him. The Lord asked Jacob his name, and he said Jacob, but the Lord said his name would no longer be Jacob but Israel, because as a prince he had power with God and with men and had prevailed. Les explains that Jacob hung tough, did not give up, and did not fail. God saw that faith and transformed him; this was Jacob's true salvation and what we today call the new birth. Since he was now a new man in God, in Christ, even back there, God gave him a new name. He was no longer to be called Jacob the deceiver, which was the natural, but Israel the prince with God. Les also reminds us not to lose sight of those two names, Jacob and his offspring, because his rule of thumb is that first comes the natural, then the spiritual, as with Cain the natural and Abel the spiritual, then Isaac the natural.
And he said, Thy name shall be called no more Jacob, but Israel: for as a prince hast thou power with God and with men, and hast prevailed.