The fact that Christians exist, and the fact that they have written about their beliefs, are proof that their beliefs are true and based on facts?
Unless you want to asset they emerged as a Jewish sect ex nihlio, they better be based on some set of facts.
Hinduism is the one true, correct religion
There is a vast gulf between asserting Hinduism is a "True Religion" and dismissing the Upanishads as counterfeit documents with no sincere authorship.
Wait, do you really think that atheists deny that Christians exist?
Atheists generally don't reject the historical existence of the Christian faith's founder.