In Islam, the only unforgivable sin is shirk, which is the act of associating partners with Allah in worship. This includes believing in deities other than Allah, attributing divine attributes to anyone or anything besides Allah, or seeking help from other beings instead of Allah.
While other sins like adultery, theft, and murder are serious, they are all forgivable if the individual sincerely repents, seeks forgiveness from Allah, and makes amends for their actions.
The Quran explicitly states:
"Indeed, Allah does not forgive associating partners with Him, but He forgives what is less than that for whomever He wills. And whoever associates partners with Allah has indeed strayed far." (Quran 4:48)
This emphasizes that shirk is the only transgression considered unpardonable in Islam. It is considered a fundamental breach of faith, contradicting the very core principle of monotheism in Islam.
