The Advice of Shaikh Saleh al Fowzan to the Sinful Caller
Question: If I have fallen into a sin and then I sought forgiveness from Allah, should this sin prevent me from calling to Allah, or should I seek His forgiveness and continue to call to Allah?
Answer: If the only person who called to Allah were a person free from all sins, no one would be qualified to call to Allah! There is no one who is free from sins, but if the Muslim commits a sin, he should seek forgiveness, and the sin should not prevent him from calling to Allah and enjoining the good and forbidding evil. The caller should not say: I have sins and incorrect deeds, I am not fit (for calling to Allah), rather, he should leave off and avoid the sins. No one has forced him to commit them. He should abandon sinning and repent to Allah, (and at the same time) he should not abandon performing righteous deeds, calling to Allah, enjoining the good and forbidding the evil.
Source: Q & A session at the end of Igathatul Lahfaan lesson 28/4/1434h. corresponding to March 19, 2013