How else do you teach stuff,
they grow up hearing its real their entire life, so they never doubt it.
I grew up hearing that the world was round; even though I have never gone around it, I don't doubt that its true.
I grew up being repeatedly told that red was red; though maybe you see it as blue, but I don't really doubt that it's red.
Everything we know is learned mostly from constant repetition, and if the teachers believe it to be true, then it will be taught.
I would guess however that:
Everyone who has ever believed in God has more than likely at some or many points doubted the truth behind their beliefs. What you are talking about is faith, thats the part which takes away the doubt.