I would think the Tzibbur is still Yotzei. But, it isn’t a very considerate thing to do. “Mitoch Sheloh Lishmah Bah Lishmah” does not apply here…
Bottom Line, if you are not a maamin AT ALL (in which case, at that point you probably wouldn’t be in the shul any longer), you don’t belong covering for the amud. Ahh, but what of one who has “doubts”? Most people do, to some extent, at one time or another.That has to be clarified on a person to person basis. Just because a person has a few questions does not mean he needs to exclude himself from davening before the amid. But, again, every person is a separate story. If he truly wants to know the answer to this question, perhaps he should go discuss his actual believing “issues” as well.