Help me understand one argument for catholicism

Some Background Information about me:I am neither an atheist, nor do i believe in any religion. I would call myself an agnostic theist (NOT agnostic atheist), as such i do believe in some kind of deity, but i don't think it resembles the deity in any existing religion. However, i acknowledge that i don't know and could very well be wrong, i just think that what i believe is likelier than other beliefs.I was born and raised in a region where, ignoring atheism, both catholic and protestant beliefs are predominant, with islam rising in popularity and slowly catching up. My family and friends were mostly atheistic or protestant, and i was raised mostly without religion. I am however very interested in religion and want to understand people's beliefs.My Problem:I have read many arguments for why people chose to be catholic, on this sub and elsewhere. Probably the most common one argued first why a creator must exist, because things have to be caused by something. I can get behind that point. The argument goes on to basically debunk all other religions, first polytheistic ones, later Islam, Judaism, Protestanthism et cetera. To be honest, i have not thoroughly looked into the reasons for why all of those religions must be wrong, and don't know if all of them make sense, so for now i will just pretend that they all make sense.So here is the part i don't understand, and i could not find anything explaining it yet:What makes you conclude that the "god" whose existence the first part of the argument tries to prove, has to be the god of an existing religion? Why is there the need for a creator to reveal themselves?If anyone can try to answer this for me, i'd be thankful.One additional, unrelated question i had regarding sins and mortal sins.It is usually stated that for a sin to be mortal, one requirement is that the sin must have been committed in "full knowledge". I have read some definitions of that, but not one that could solve this question:If someone knows that Catholicism considers something a mortal sin, but does not know/believe that catholicism itself is true, would that still be full knowledge?Thank you very much.