2 Questions on Islam

Because we are follower of true religion and it's mandated by the religion to tell others about it.

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2. Why is a non muslim referred to as a kaafir?
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I think in pre-Islamic times word infidel/kaafir was only used for the idol worshipers. By the time or probably it's started from sub-continent now it's a general term for all the non-Muslims. Wallah-o-Alam !