Asalam o Akaikum!!! When people say that something is not a sin(but that thing is a sin whether major or minor).they say such on lack of knowledge or due to wrong traditions of daily life, or knowing that it is a sin but they just say its not a sin and start giving different statements to justify themselves.what's ruling on these people who don't say a sin that it is sin. How should we deal with these people? What's ruling on person who legalize a sin and what exact involve in legalizing a sin?

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.

It is incumbent on a Muslim to avoid all forbidden acts, major and minor alike. Verily, a minor sin becomes major by committing it with insistence. Meanwhile, a major sin is completely erased by seeking forgiveness from Allaah. However, a Muslim should beware of the evil schemes of the (evil-inclined) self which deceives the person to deem what Allaah has deemed unlawful lawful so that he would easily commit such forbidden acts with no sense of guilt or remorse.

It should be noted that sinners should be (kindly) advised and guided to the straight path and this entails adhering to patience, wishing for their being guided, and highlighting the Truth for them.

In fact, deeming lawful what is unlawful means to believe that such prohibitions are allowable. This attitude may take the person out of the fold of Islam if such prohibited acts are agreed upon by scholars and their prohibition is well-known to lay Muslims. Ibn Qudaamah wrote, “...It is an act of disbelief to hold that some act is lawful although it is deemed prohibited based on unanimous scholarly agreement, and its ruling is evident to all Muslims without any misconceptions or doubt involved concerning its prohibition, given the reported religious texts about it, such as the prohibition of eating pork, adultery, and similar acts whose prohibition is established without disagreement in opinions.” [Al-Mughni]

In any case, it is impermissible for a Muslim to label other Muslims as disbelievers without taking into consideration the due conditions and criteria governing that serious and grave matter. Accusing a Muslim of disbelief means taking him out of the fold of Islam, which entails many rulings in this worldly life, aside from the consequent implications pertaining to the Hereafter.

For further information, please refer to Fataawa 8106, 19773 and 91110 about the due conditions and criteria that should be considered before labeling a Muslim as a disbeliever.

Allaah Knows best.