I will sing unto the Lord, because he hath dealt bountifully with me. To believe in the Lord Jesus is to understand who He is what He has done for you.
Man is sinful.
He created everything, including man, and out of His great love gave us free will. He is so holy and so perfect that any inperfection can not exist in His presence. God is holy and perfect - you can not be with Him after death if you are a sinner. God the father sent His only son, Jesus who lived a perfect life on earth, without sin, to become the only acceptable sacrifice for our sins. He just needs you to acknowledge what He did on the cross for you, and receive Him in your heart.