Joel 2:13

Rip your heart to pieces (in sorrow and contrition) and not your garments. Now return (in repentance) to the Lord your God, for He is gracious and compassionate, slow to anger, abounding in lovingkindness (faithful to His covenant with His people); and He relents (His sentence of evil (when His people genuinely repent). 

The door to God is always open to the person who genuinely repents in their heart. The devil will tell people they can never be forgiven, and he is a liar. If we make a mistake (and we all do), repent and get back in fellowship with God. Bar the door to the past mistakes and turn away from the old temptations. Remind them that God is the savior and made us free from their influence.