The most serious thing that Allāh has forbidden is shirk (associating partners with Him). Shirk means worshiping or attributing divine qualities to anyone or anything other than Allāh. This includes practices like praying to others besides Allāh, seeking help from idols or false gods, or attributing Allāh’s unique qualities to His creation.
Allāh says in the Qur’an:
“Indeed, Allāh does not forgive associating partners with Him, but He forgives anything else for whom He wills.”
— Surah An-Nisa (4:48)