I don’t think actually believing the views you defend is relevant here. Playing devil’s advocate can be done in good faith. It’s about your intentions. In fact, I’d argue that being able to clearly articulate a view you don’t hold is a sign that you’ve genuinely understood your opposition’s arguments. You don’t need to be convinced by them yourself.

What does make it bad faith is if you put those arguments forward but then refuse to engage with the counterarguments - that’s where the line gets crossed.

For example, I don’t agree with the reasons Russia has given for attacking Ukraine, but I can still lay out those arguments in a way a pro-Russian person would recognize as accurate. That, on its own, isn’t bad faith. But if someone responds by calling me a delusional Nazi or something similar, that is bad faith - an ad hominem, specifically - even if that person genuinely believes people who argue that position deserve such a label.