The Believers - 23:117
And whoever worships another god besides Allah, who has no proof, his reckoning is with his Lord. Surely the disbelievers cannot succeed.
Allah does not forgive those who serve other than Him:
The Women/4:48
Allah does not forgive associating partners with Him, but other than that He forgives to whom He wills. Whoever associates partners with Allah has committed a great insult.
The Women/4:116
Allah does not forgive associating partners with Him (if it continues until death), but He forgives the sins below that to whom He wills. Whoever associates partners with Allah has gone astray.