It's very believable. I'll give a couple of techniques here.

• Reinforce skeptical behavior while modifying their self-image to that of a truth-seeker: "I love talking to you because you pursue the truth over comforting lies". Be genuine, and by that I mean use the tone you would use to tell someone that their suit looks good.

• Give high-status cues. Assume the role of teacher or mentor. Once they want to become more like you, merely expressing your beliefs (not opposing theirs, but expressing yours) will make a significant impact.

• Demonstrate that giving up religious belief won't result in isolation. How exactly you do this will vary based on the social context.

There are a couple others, such as generating low-status associations with religion, which is a bit advanced and so not worth covering here, and creating false memories and comittments, which is scarily easy to do but absolutely dark arts and therefore not covered here.

You will note the absence of "rational argument" on this list. That's because rational argument is rather ineffective for changing the mind of the person you are arguing with (though it may change the views of observers).