What is meant by the act of deeming lawful (istihlal) that expels the perpetrator of a grave sin from the fold of Islam? Is it limited to verbal utterance, or is persistence in the act while acknowledging its prohibition considered deeming lawful? What is the ruling on one who persists in sin, knowing its prohibition and loving it? Does this contradict faith?

Istihlal (considering something unlawful to be lawful) is the belief that what Allah has forbidden is permissible. Its ruling varies depending on the forbidden act:

If the forbidden act is something upon which scholars have reached a consensus and its prohibition is established by definitive texts (like drinking alcohol and fornication), and the one who deems it permissible is aware of that, then it constitutes disbelief (kufr).

If the istihlal pertains to a forbidden act about which scholars have differed, or if the perpetrator is excused due to ignorance, a misconception, or being a recent convert to Islam, then it does not constitute disbelief.

Insisting on committing a major sin while believing it to be forbidden is not considered istihlal and does not lead to disbelief; rather, it is an act of transgression (fisq) and a weakness of faith. And whoever commits a sin out of love for it, while believing in its prohibition and hoping for repentance, is a transgressor. However, whoever commits it out of aversion to the Sharia or arrogance towards it, this is disbelief.

Sins may gradually drag their perpetrator towards disbelief.