When you actually believe Jesus, you actually do what he says.
...only unto him that has faith unto repentance is brought about the great and eternal plan of redemption. (Alma 34:16)
Professing belief in God isn’t sufficient to be saved. You can profess belief in God and still sin:
They profess that they know God; but in works they deny him, being abominable, and disobedient, and unto every good work reprobate. (Titus 1:16)
Jesus can only forgive those who stop sinning:
Who am I that made man, saith the Lord, that will hold him guiltless that obeys not my commandments? (D&C 58:30)
God’s sacrifice doesn’t make you free to sin—it is meant to make you free from sin. God’s sacrifice makes it possible for bad people to become good people, for sinners to become saints, and for mortals to become more like God.