 Sin had to be paid for because God's a God of perfect righteousness, and he hates sin. There's no getting around that. But God, in his love, allowed Jesus to pay that righteous payment in our place, in full, on that cross. But that doesn't become ours until we, by faith, reach out and receive that payment. God said that it's a free gift. Ephesians 2.8.9 says, For by grace are you saved, that's saved from hell, through faith, and not of yourself. It's the gift of God.