For the wages of sin is death; but the gift of God is eternal life through Jesus Christ our Lord.
Romans 6:23 (KJV)
For many of us, we must first understand what it means "the wages of sin is death". By sin is meant any sin, original sin, actual sin, any type of sin, lesser and greater: so the wages, which is death, is what we earned, what we deserve. Death is not just physical as we know or think it is, it's also spiritual and eternal.
Our sin was an offense to God, the punishment was death. But God, so gracious and merciful, offers eternal life as a free gift through the blood of Jesus Christ. As the book of Romans clearly indicates, we receive this gift by faith in Jesus Christ, that is, by believing in His death for our sins and presenting our lives to him, to serve him as our risen Lord and Saviour.
God determined that there would be only one way to be forgiven of sin; one way to be redeemed; one way of being declared righteous; one way to obtain eternal life. And Jesus told us: "I am the Way, the Truth, and the Life; no one comes to the Father except by Me."
No man can gain eternal life and enter heaven without having faith in Jesus.