Blurring the lines.

[FatAndFaithless]

Because there is literally one definition of atheist, not a code or set of beliefs or a canon. Saying you're a god-believing atheist is like saying you're a married bachelor, it's not a question of doctrinal differences or anything vague, it's literally a definition of one stance on one question for one person.

It's very easy to "test" someone to see if they're an atheist or not.

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Yeah, it's a really easy test actually.

"Do you hold a belief in a god?"

"No"--> Atheist

Anything else --> non-atheist

There is a very simple test to determine whether one is a Christian or not. 1st do the claim to be? 2nd do they follow the canonical teaching of Jesus Christ?

Those are the only two requirements Per the Apstole Paul, and his letter to the Galations.

Why are their requirements for atheism who has no rules, and no requirements for those who claim Christianity who does have rules?

Because, you unbelievable dolt, there is a massive disagreement among Christians as to what constitutes your 2nd condition.

"The canonical teaching of Jesus Christ" is something that no two sects agree on (which is why they are two different sects in the first place), not to mention that some sects (Catholic Church, for example) don't place the literal scripture in the Bible as the paramount important deciding factor of Christianity. Shockingly, people in your own religion disagree with you, and until you can actually prove them wrong or why you have the magic true checklist, you can take a step down from your self-appointed high horse.