First of all, taxing religious groups is a very quick way to put smaller churches and groups with already thin margins under water. This would force them to consolidate with the already extreme mega churches who could afford to take the hit.

I'm pretty progressive fiscally, but this is my main reason against supporting taxing churches. Lots of people conceptualize churches as massive, with thousands of congregants and led by ministers (I use that term somewhat loosely) who make six figure salaries by preaching fundamentalism from the pulpit and telling their congregants that they will be saved if only they donate more to the church. And there are some congregations like that, especially if you're in certain parts of the country.

But there are also many churches that are much smaller, get maybe 100 people on any given Sunday, have leaders and congregants that are decidedly not fundamentalist in outlook (that's often the appeal; in fact, many are quite progressive), and are often struggling to make ends meet. Look closely some time next time you drive through a medium sized city. I bet you can find at least one church building that's pretty old looking but has a rainbow flag or BLM sign out front. I live in the American South, and there are several within a few miles of me.

Taxing churches may make some money off of the former, but it will put many of the latter under for good. I'm all for socializing healthcare, but there are corporations run by the mega wealthy (Amazon comes to mind) that can better afford it. You'd accomplish the same goal without harming the spiritual life of thousands of people (many of whom support socialized healthcare themselves).