Can somebody explain me why the following sentence is incorect?

Double negatives - 'not' and 'never'.
One negative is enough. You could say, "I will not betray God, ever." Alternatively, in conversation or dialogue you could say "I will not betray God. Never!"

It should be ever not never. The use of never makes it a double negative.

It suggests a double negative, in other words, it suggests that the word "never" is a response to the first part. It's not like it's incorrect, it's just that the word "ever" is better in this sense because you avoid the suggestion of a double-negative.

Because God doesn't exist, so how can you betray someone who doesn't exist?

I will not betray God ... ever!

ever, because you have already stated I will not