Jesus told us that if we hate someone in our heart we are guilty of murder, but no one has been sent to the electric chair for hating another person. So, let's rephrase the question. Instead of asking what is the greatest sin that a Christian can commit, we need to ask what is the most destructive sin that a believer can commit. In my experience, the most destructive of all sins is bitterness. Bitterness destroys people, families, and churches.
Bitterness is an attitude that refuses to forgive offenses.
Like a cancer, it grows until it destroys everything around it. Forgiveness is an attitude that honestly acknowledges an offense and then dismisses it on the basis of God's forgiveness of us. Bitterness holds on to an offense, but forgiveness brings release. So tonight the challenge is to choose forgiveness over bitterness. We can choose to forgive when we understand the basis for forgiveness.