While they were eating, Jesus took some bread, and after a blessing, He broke it and gave it to the disciples, and said, “Take, eat; this is My body.” And when He had taken a cup and given thanks, He gave it to them, saying, “Drink from it, all of you; for this is My blood of the covenant, which is poured out for many for forgiveness of sins. Matthew 26:26-28
Forgiveness cost Jesus His life. He approached the cross willingly for love. No greater love is there than that someone would lay down his life for his brother/friend. Jesus laid down His life for His enemies.
To not forgive someone when you are a Christian, to me, would be like trampling on the blood of Jesus. It's not a minor thing. It's serious. Forgiveness if bought with Christ's precious blood and to shed that blood, he went through agony. He was simply shot with a bullet or speared through.
He was mocked, spit upon, slapped, called names, ridiculed, made fun of. He was beaten and tortured and if all that wasn't enough, He suffered the worst death, death on a cross.
To not forgive, is trampling all over what Jesus has done on the cross for the forgiveness of sin.