Can SOMEONE explain to me WHY CHURCH & POLITICS SHOULD BE SEPERATE??

We have these laws because of the marriage of religion and churches in European countries like England,Russia,Italy,etc the churches were using the government to force religious ideas on people.Citizens can vote just like anyone else whether they're in their church community or not.They don't need the churches speaking for them.And the churches get a tax free exemption because of their status.Do you want the government telling churches who should or should be members of their churches and what it should be based on.How to conduct church business like donations or education.Stay out of my government business and I will stay out of Church business.But citizens can vote based on their own religious ideas if they want to.Churches can criticize the government if they want but that would still be a citizens opinion because I have never been to a church were everyone is like minded all the time.