Jumping back to Kate Forbes for a moment - Ian Blackford, until recently SNP leader at Westminster, is also Free Church. He has seemed to tow the Sturgeon-mandated SNP "progressive" line, and until recently when he was removed, rumours of discord have mostly been about him wanting to allow more dissent and diversity of thought than Sturgeon wanted.
There was a small kerfuffle after the UK Parliament voted through a raft of policies for NI including same sex marriage and limited abortion rights, during the period when NI had no functional government/parliament. Tabloids claimed that Blackford's church (the individual church he attended, not the denomination) had asked to speak with him about his vote FOR the bill. Blackford refused to comment, and the church quickly went silent, saying it was a private matter. Forbes was asked about the NI vote (hypothetical, as she's an MSP not an MP) and said she'd have abstained because the UK shouldn't legislate for NI in devolved matters. (Which was originally Sturgeon's position, although in the end she did allow a free vote.)
IF Forbes does enter the leadership race, I think the key is whether she's willing to say that as FM (and SNP leader) she would, if needed, be able to put aside personal views on (for example) abortion provision and support or even champion a policy at odds with her faith. She has previously said that she's in politics to represent her constituents not to push her own beliefs - but as others have said, there's a big difference between voting the party line when whipped, and being influential in setting the party line. If she does stand, I'm sure she'll be asked detailed questions on this at hustings and by the press.
I don't want to put words in Forbes's mouth, or Blackford's, so let's say some hypothetical candidate emerges who actually DOES say "I want to reduce abortion rights and limit same sex marriage". I would not personally vote for this position, BUT is it different in principle to Patrick Harvie's or Mhari Black's deeply heartfelt promises to remove and criminalise women's spaces and services ? Or Rishi Sunak's or Boris Johnson's to uphold hypercapitalism at all costs? Historically we've viewed faith as distinct from ideology, but if people like Harvie literally CANNOT listen to and consider another view, are faith and ideology (in this case) relevantly different in the way they impact constituents, and the country?