Assalaamu Alaykum. I know that saying that a Muslim is a kaafir (disbeliever) is a grave sin and that the kufr (disbelief) reverts back if the other person is not kaafir. But does kufr revert back if someone ask the other Muslim whether he is a Muslim or a Hindu or when the person asks the other person whether he is Muslim or not? Or when they say, 'Do not become kafir (without clearly stating that he is a kaafir)?' The Muslims who say such phrases to other Muslims do so because of their wearing indecent clothes, not praying, and other unislamic things... Many Muslims say these phrases to others.

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 is His slave and Messenger.

It is impermissible for the Muslim to accuse his fellow Muslim of kufr; Ibn ʻUmar narrated that the Prophet said, "Any person who called his brother, 'O kaafir' (has in fact done an act by which this unbelief) would return (bounce back) to one of them. If it were so, as he asserted (then the disbelief of the addressed man was confirmed, otherwise it returns to him (to the man who called his brother Muslim as such)." [Al-Bukhari and Muslim]

This does not mean that such a person, who labeled his Muslim brother as a disbeliever, has committed an act of disbelief that takes him out of the fold of Islam; rather, it means that he has labeled himself a disbeliever when he labeled his Muslim brother as such.

Tuhfat Al-Ahwathi reads:

"The hadeeth means that the Muslim who labels his Muslim brother a disbeliever has actually labeled himself a disbeliever as well because they are both similar (i.e. they are both Muslims). In this case, the takfeer (declaring a Muslim individual or a group disbeliever), not the Kufr (disbelief), bounces back to the person who declares his fellow Muslim a disbeliever because it is the same as declaring himself a disbeliever since they both are Muslims or because he has done something that only a disbeliever who disbelieves in the religion of Islam would do..."

As for simply asking the Muslim person whether he was a Muslim or offering him a piece of advice, this does not count as labeling him as a disbeliever nor does it fall under this heading since he has not accused him of disbelief or described him as a disbeliever.

The Muslim should advise the sinners or those who fall short in adhering to the teachings of Islam wisely and kindly; he should choose the optimal and wisest words of advice that would not drive the sinner away from obedience to Allaah or cause him to develop an aversion to the advice. Allaah, The Exalted, says (what means): {Invite to the way of your Lord with wisdom and good instruction, and argue with them in a way that is best...} [Quran 16:125] Moreover, the Prophet said to 'Aa'ishah "O 'Aa'ishah! Verily, Allaah is Compassionate and loves compassion; He gives for compassion what He does not give for harshness." [Muslim]

Allaah knows best.