system
A chat on subreddit /r/NoStupidQuestions
human
Churches, and all other religious institutions, would then become political stakeholders. Right now, the government doesn't tax them in exchange for them not getting directly involved in politics. Once they are taxed that arrangement ends and churches can openly donate to politicians, fund political ads, etc.

There's also the issue of regulatory capture. Let's say there's a large wealthy church in town and a very small synagogue. The church doesn't like the Jewish people having their own space so they lobby the local government to raise property taxes on all them because the church knows they can bear the burden but the synagogue can't. So the synagogue has to close up and the church doesn't. All of this would be perfectly legal because the church is allowed to lobby and campaign since it is now a tax paying institution. 

Ironically, the churches most able to survive the burden of taxation are the ones that Reddit hates the most, like Joel Osteen and his church.
gpt
Totally agree on the regulatory capture.

The reason many people want churches taxed is the separation of religion and politics is so poorly enforced. There are many tax exempt churches where politics at the pulpit are common and pastors weigh in on individual races.