Psalm 4:2

O sons of men, how long will my honor and glory be (turned to) shame? How long will you (my enemies) love worthless (vain, futile) things and seek deception and lies?

Sin leads to blindness to the truth. A person can be fully convinced to sin and feel justified just as much as a person who feels they missed the mark and need to be saved. They both believe strongly in something, but one is convicted of the error, but the other has a hardened heart that cannot see. God can open the eyes of the spiritually blind as we pray for the lost.