What religion did Muslims not successfully conquer?

Answer 1

England apex

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Muslims never conquered any religion. It was the teachings of Islam religion just to free the will of people to be able to practice the religion they are convinced with. It is a basic principle in Quran that no compulsion in religion.

Quran says (meaning English translation):

"There shall be no compulsion in [acceptance of] the religion. The right course has become clear from the wrong. So whoever disbelieves in Taghut and believes in Allah (God and same God in Christianity and Judaism) has grasped the most trustworthy handhold with no break in it. And Allah is Hearing and Knowing." (2:256)

Answer 2

The question as posed is nonsensical, Islam is a religion, which means that it is an ideology, customs, traditions, etc. but not something physical. As a result, it cannot conquer anything. Muslims have conquered territories that were previously majority-Zoroastrian, majority-Christian, majority-Hindu, majority-Buddhist majority-Sikh, majority-Animist, and majority-Polytheist. They also conquered regions with large numbers of Jews (but there was no place that Muslims conquered that was already majority-Jewish because there were no such places prior to Ottoman Thessaloniki). All of those religions that existed in territories that Muslims have conquered still remain to this day. Admittedly, Zoroastrians are not doing terribly well today, but that is more an issue with the Safavid Empire in the 1500s-1700s.