Will God forgive a person who dies upon a major sin, such as one who habitually commits adultery and dies before repenting, even though he knew of his sin and delayed repentance?

Adultery is a grave sin, one of the major transgressions, as indicated by texts from the Quran and Sunnah. For example, Allah Almighty says: "And come not near unto adultery. Lo! it is an abomination and an evil way," and the Prophet (peace be upon him) said: "The adulterer is not a believer while he is committing adultery," and he said: "When a man commits adultery, faith departs from him and hovers over him like a cloud, but when he stops, faith returns to him." Allah has promised severe punishment in the Hereafter for adulterers, as mentioned in the Prophet's vision (peace be upon him) about male and female adulterers being tormented in an oven. The punishment for adultery in this world is one hundred lashes for the unmarried virgin and stoning to death for the married person (muhsan).

Whoever has committed adultery must hasten to repent and feel remorse, and cut off all connections that lead to it. For whoever repents, Allah accepts their repentance and transforms their bad deeds into good ones, as stated in Allah Almighty's saying: "Except those who repent and believe and do righteous deeds; for those, Allah will change their evil deeds into good deeds." As for whoever dies upon adultery without repentance, their matter is up to Allah, subject to His will, but one must beware of underestimating Allah's punishment.

Sins in general, and adultery in particular, affect and weaken faith, and may lead to a bad ending, for sins are the courier of disbelief.