Answer:

Allah does not forgive those who set up partners with Him; but He forgive to whom He pleases; for anything else; to set up partners with Allah is to devise a sin most heinous indeed. (Quran: 4:48)

Allah does not forgive the sin of associating other gods with Him; but He forgives whom He pleases for sins other than this: one who associates other gods with Allah, has strayed far, far away (from the right). (Quran: 4:116)

Answer:

But it has already been revealed to you, as it was to those who went before you, “If you were to associate other gods with Allah, truly fruitless would your work in life be,” and you would surely be in the ranks of those who lose all spiritual good”. (Az-Zumar, 65)