Is one who says, "I am not afraid that Allah will punish me because of my sins," an unbeliever?

Feeling secure from Allah's plan is one of the major sins and can even amount to disbelief (kufr). If the fear is entirely absent from a person's heart, that would constitute disbelief. However, if their statement is due to the intensity of their hope and a weak deterrent of fear — while the root of fear still exists in their heart — then they are among those who commit sins.