And if God chose his people by grace, then it is not the things they have done that made them God’s people. If they could be made God’s people by the things they did, then God’s gift of grace would not really be a gift.
People are made right with God by his grace (kindness). This is a free gift. People are made right with God by being made free from sin through Jesus Christ.
Why not? Because they tried to make themselves right by the things they did. They did not trust in God to make them right. They fell over the stone that makes people fall.