I believe that it is very hard for an atheist to convert a theist because an atheist would use reasoning and logic in their argument. Since theists cannot comprehend reasoning and logic it would be like speaking a different language.
An atheist must first encourage the theist to use proper logical reasoning, and then attempt to de-convert/convert/convince or whatever.
An atheist must first encourage the theist to use proper logical reasoning, and then attempt to de-convert/convert/convince or whatever.