The only other way to know about their religion is if you practiced it. DID YOU?!?
Did you ever think before you post?
Let me list for you some alternative ways
I studied their religion
I have a neighbor who practises that religion and we argue religion all the time
Same as above but with a coworker
I heard shiurim from my Rebbe on the topic
And perhaps I did practise it. So what? Maybe I am a ger tzedek and you were just מבטל מצוות עשה של ואהבתם את הגר
Maybe I am a בעל תשובה and you were just עובר ולא תונו איש את אמיתו
”And most Frum Yidden, call Hatzola in an emergency, Not 911, doesn’t make them right either!”
Doesn’t that sort of disprove your point?
Are you now going to respond to yourself with some nasty gibberish?