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The easiest way is to simply copy an existing religion and then do your desired changes from there.
In religions.txt at the top is a description what the lines in that file mean.
 
Is that the only file that has to be changed? Or could you give more details what has to be done and where?
If you add a new religion to religios.txt, e.g by copying and editing an existing religion, then that religion exists in the game - but no province has the provincereligion and no country that religion as starting religion. That has to be changed in other files
 
You'll also want to make an icon for the religion in Gfx\Religions, otherwise it will look weird in game. The .bmp file name should match the entry in religions.txt. Then of course you'll also want to put a text string for the religion into a localization file.