When we commit sin, we put up a wall between ourselves and God. God loves us deeply, but He hates sin so much that it causes separation between man and God. Our sins are so great, that we cannot possibly repay them. The good news is that His love for us is so great, that He sent His only begotten Son to die on the cross as payment for our sins. The only way our sins can be forgiven, is when we receive the gift that His Son has given us on the cross.
Forgiveness and reconciliation is the most important part of the gospel. Without forgiveness of our sins, we face eternal separation from God. Anybody who dies who is separated from God, goes to a place of eternal punishment called hell; a place where there is no relief from the Lake of Fire (Revelation 20:15).