Simple. Christians have celebrated Xmas for millenia. What differentiates our Holidays from theirs?
As a Jew you believe that their Holidays have no real basis. What makes you think ours has? They celebrate their Holidays because their parents celebrated it and their grandparents before that. How do we know that ours is any different?
Can you independently verify that our festivals have a real basis? Let’s start with Chanukah. FM went on and on for years about how there is no “documented history” regarding the miracle of the oil on Chanukah. Why is the story of Purim not well known to historians? If Hashem did fantasy-style miracles for the Jews in Egypt, why did he stop now? Surely he could have hit the Nazis with at least frogs or lice?
Sorry, I don’t want to preach kefirah here but you get my point. Emunah is the entirety of it.
My grandfather and farher arent liars. If you only have emunah as your failsafe from discounting Judaism as just another religion then its a very thin line to going off.
To follow your logic why have emunah in Judaism. The same way Moshe Rabbeinu came down from the mountain with the Torah, lehavdil elef alfei alafim, Mohammed heard an "angel" speak to him in the cave and gave him the Koran.
What is emunah? Pick and choose what sounds best to you? Of course not.
Judaism is the only religion that has a mesorah of national revelation, ma'amad Har Sinai. My grandfather and his grandfather have the tradition all the way back to that time. We heard אנכי. No other religion has that mesorah. They have individuals who claimed to have received messages.
So you will tell me Moshe Rabbeinu also made up everything besides the עשרת הדיברות. That is where אמונה and the 13 Ikrim come into play.