No, a lot of Christian stuff is made up too. Fasting in Lent, Advent, Catholics praying to Mary. None of it Biblical. Plenty of other examples.
And why is it a stretch to think that what I believe is true and, say, Judaism and Islam are not?
They contradict each other. They can't all be true. They could all be false of course but either one is true or none are. Logically there is no other possibility.