Atheists, tell me, a Roman Catholic: why should I become an atheist?

[Mudhammam]

Atheists, why do you reject the idea of God, and why should I? I know that your answers will include "there's no evidence" and all that, but please try to explain.

1.  The general concept of "God" is usually poorly defined. More specific conceptions of God, such as the Trinitiarian "deity" of Roman Catholicism, tend to be self-contradictory and incompatible with the nature of reality (for example, the problem of evil).
2.  No specific statements about God or alleged interactions with terrestrial beings are more likely to be true than any other (contrary) statements.  Hence, if 99.9% of all other (e.g. non-Catholic) gods must be considered false given the evidence or arguments put forward for their existence, and the .1% of faith claims (e.g. those within Catholic orthodoxy) that are believed to be true have nothing exponentially greater in terms of the evidence or arguments put forward for them, epistemologically we are only in a position of saying that either (A) all 100% are true or (B) all 100% are false.  Since (A) entails self-contradiction, the most probable conclusion is (B).
3.  The claims about God are on their face no different than the sorts of claims that cannot in principle be proven true or false, and moreover, boast of no pragmatic value that is specific to their being true (in this life, at least); or, if they can in principle be proven true or false, they lack sufficient data to declare with certainty one way or another (as is the case with big foot, alien abductions, ghosts, etc.); given the contradictory nature of competing claims, probability demands that each rules out every other (see #2).
4.  Lack of evidence in support of theism coupled with a growing body of evidence showing human beings to be immensely ignorant of the causes underlying their immediate experiences, and the fact that it typically involves claims about the world that can be traced to primitive notions of which even most believers find too incredible to give their assent (e.g., Christianity can be traced to Hellenistic Judaism and Greek philosophy, which are themselves mostly derived from ideas that originate with more ancient religions, e.g. Egyptian, Sumerian, etc.).  All of this, to say nothing of the specific faith claims themselves, screams out contrivance.