I know a woman who works in the field of Da'wah (Islamic propagation). This woman is a very successful caller to Allaah The Almighty and has a very positive effect on other women. However, when I sit with her in private sessions, she commits one of the most heinous sins: she backbites everyone including her own parents; she criticizes them bitterly and shows no respect to them in front of others.
How can she be a successful caller to the worship of Allaah The Almighty, and a humble worshipper who weeps in prayer when she commits such terrible sins? Could you explain?

All perfect praise be to Allaah, the Lord of the worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad, sallallaahu ‘alayhi wa sallam, is His Slave and Messenger.

No human being is perfect. Perfection is a divine attribute. No person in this world is safe from committing sins and making mistakes.  However, when we commit a sin or make a mistake, we must repent, and ask Allaah The Almighty for forgiveness.  The Prophet, sallallaahu ‘alayhi wa sallam, says that all the sons of Aadam (Adam) make mistakes, but the best of those who make mistakes are those who frequently repent.  [Ahmad and At-Tirmithi]

The Prophet, sallallaahu ‘alayhi wa sallam, also said that if we never committed sins, Allaah The Exalted would replace us with  people who would commit sins and then seek forgiveness from Him, and He would forgive them. [Muslim]

At the same time, this should never suggest that we are encouraged to sin. Indeed, we must show the love of Allaah The Almighty for those who repent seeking His forgiveness, and Allaah The Almighty accepts their repentance.

The sister who works in the field of Da'wah, yet backbites others must be a good example to others by practicing what she preaches.

On the other hand, a Muslim who sees her commit such sins  should advise her and remind her with the Quranic verses in which Allaah The Almighty Says (what means):

•       {O you who have believed, why do you say what you do not do? Great is hatred in the sight of Allaah that you say what you do not do.} [Quran 61:2-3]

•       {Do you order righteousness of the people and forget yourselves while you recite the Scripture? Then will you not reason?} [Quran 2:44]

And Allaah Knows best.