This is is the first counter argument I use:
There are 4,200 religions in the world have you studied all 4,200 before concluding that your God is the true god and the rest are false prophets created by man?
Why are other Gods false and yours not,if other Gods are false shouldn’t yours be too?
My experience has been almost exclusively with Christianity and the following responses are my experience with Christians of many stripes.
Believers have much personal experience that they attribute to their own God. This being the case, they have much reason to believe their own God is the true God and all other gods are false gods. The logic goes this way: If my God were not real, this would not have happened to me—it could not have happened. That is how I know God is real.”
Others will point to their sacred text and argue that it is true because they just know it’s true. They might challenge anyone who disagrees with them, even attacking their intelligence or ethical stance on the claim that all knowledge and morality originates with their own God.
“It’s faith and I don’t have to substantiate it,” is another defence they might come up with. Faith, as I’m beginning to understand, is believing without knowing and trusting without understanding. In other words, it means accepting as truth that for which there is no evidence or reason to believe other than the word of authority. I suppose this is why most people adopt the religion of their parents. I’m an atheist because “God” did not give me the gift of faith.
As for the “many religions” and “false gods” charges, believers will most likely discount it as so much nonsense from an ignorant atheist.