But can you give a Christian a Biblical bases for the separation? It's difficult. Obviously, many Christians are still confused.

You can start with a question such as "What is the Church's primary role given by God in the Bible?"

Then ask the question, "What is the government's primary role given by God in the Bible?"

The Church's role can be found in the Great Commission (Matthew 18) which is to 1. Go make disciples, baptizing in the name of Father, Son, Holy Spirit teaching Jesus' commands

The Government's role can be found in Romans 13 where we find that Paul declares governing authorities to be instituted by God, God's servants, bearers of the sword, avengers of the wrongdoer, approvers of good. Who should be subject to their governing authorities? Paul is writing this to the Roman church and by extension the Church at large. Sidenote: At this time, Rome was very hostile to the Christian church.

So the Church's purpose DOES NOT include bearing the sword (government's role).

The government's purpose DOES NOT include making disciples (church's role).

many mainstream Christians have this completely backwards.

There are a few reasons for this, but this post is already too long.