If he became acceptable to God because of what he did, then he would have something to boast about. But he would never be able to boast about it to God.
Even before they were born or had done anything good or bad, the Lord told Rebekah that her elder son would serve the younger one. The Lord said this to show that he makes his own choices and that it wasn't because of anything either of them had done.
God saved us and chose us to be his holy people. We did nothing to deserve this, but God planned it because he is so kind. Even before time began God planned for Christ Jesus to show kindness to us.