"There is no compulsion in religion..." (Quran, Chapter 2, Verse 256)
This statement above is supported by another Quran saying (with meaning English translation):
"...If God had willed, He would have made you all in one nation and one religion), but that He (God) left you in different religions to compete in good deeds and society benefits. The return of all of you is to God; then He (God) will inform you about that in which you differ".