given some real, non-zero value, a
a = b
a*b = b2
a*b-a2 = b2-a2
a*(b-a) = (b-a)*(b+a)
a = b+a
This is obviously wrong. You don't even need to understand the algebra to see that it's wrong. If a is not zero, and a equals b, then a cannot equal a+b. The argument is nonsense. Identity is not a conclusion. It is an axiom. It is foundational to mathematics.
As far as theists are concerned, the same thing applies to their god. If your logic leads to the conclusion that their god doesn't exist, then, to theists, there is obviously something wrong with your logic; even if they can't say what that is, exactly. For them, their god is an axiom, foundational to reality. Only when a theist is able to reject that premise as false can they be reasoned with.
That is why I stick to "Evidence or STFU" when someone challenges my atheism.