I don't know which...

[99bottles]

I don't know which religion is the correct religion or whether there is a God to begin with/at all.

Which idiom sounds correct/natural here?

 

[Tarheel]

I think "at all" is the better choice. However, it's more natural, I think, to say, "I don't know if there even is a God."

 

[emsr2d2]

"to begin with" doesn't work there. I agree with Tarheel that "at all" is the better of those choices. His suggested version is even more natural but note that you will hear native speakers put "even" in various positions in that sentence.

I don't even know if there is a god.
I don't know if there even is a god.
I don't know if there is even a god.

 

[99bottles]

"to begin with" doesn't work there. I agree with Tarheel that "at all" is the better of those choices. His suggested version is even more natural but note that you will hear native speakers put "even" in various positions in that sentence.

I don't even know if there is a god.
I don't know if there even is a god.
I don't know if there is even a god.

Wait, should I replace whether with if? Is whether wrong?

 

[emsr2d2]

Wait, should I replace whether with if? Is whether wrong?

Sorry, I didn't even notice that you'd used "whether". I naturally used "if" but your original isn't wrong.