And sure, you might derogatorily refer to someone’s religion as a superstition, just like I might call someone a “pagan” even if he’s strictly speaking just a heretic.
But in reality these are all precise terms with different meanings.
To be a bit more precise, superstitions deal with more or less automatic things like receiving luck by touching some rock. If you swear to God, that is not some automatic process that will bring you luck, its a divine being with a will that presumably would be angry if you break your oath. Do you see the difference?