What is the difference between a Muslim who is truthful in every word and a Muslim who is untruthful in every word?

The difference between the truthful and the liar is like the difference between the obedient and the disobedient. Truthfulness is a sign of faith, and lying is a sign of hypocrisy. The Prophet, peace and blessings be upon him, commanded us to be truthful and warned us against lying. Truthfulness leads to righteousness and Paradise, while lying leads to wickedness and Hellfire. A true Muslim cannot be a liar.