Imagine a person who visits a sorcerer even when they know that it is a form of shirk in Islam. If this person does not sincerely repent to Allah, then they continue to commit that sin till they basically believe in the sorcerer more than they believe in Allah and His Messenger.
“The Prophet (ﷺ) [said] about the major sins: “Shirk with Allah, disobeying the parents, taking the life, and false statement.” (Tirmidhi)