In Matthew 11:11, Jesus states:
"Verily I say unto you, Among them that are born of women there hath not risen a greater than John the Baptist: notwithstanding he that is least in the kingdom of heaven is greater than he."
I understand Jesus to be teaching at least three key ideas here:
John had a faith / faithfulness that was greater than anyone who had previously lived (other than Christ, that is);
Despite John's spiritual "greatness" on earth, all the saints in heaven are spiritually greater than John (who was still in an imperfect condition on earth). John still had a sin nature; he was subject to sinful thinking. I praise God that, because of Christ, we have the certain hope of living in a perfected condition in heaven!
The phrase "he that is least in the Kingdom of heaven" seems to refer to the different degrees of reward in heaven (for service on earth). The saint who receives the smallest reward in heaven will still live in a spiritually-perfected condition. Therefore, that Saint in heaven is spiritually more perfect than was John (who was still on earth).
Needless to say, we should aim for spiritually "great" service on earth (humble, obedient service) so that we may enjoy a great eternal reward in heaven. Both will glorify our Lord!
Lord, help us to faithfully serve you "with all humility of mind" (Acts 20:19).