I was wondering if this is to be taken as doctrine (meaning that the government should always be separated from the Church), or if it should be taken as advice on what to do if government is not united with the Church.
We all know that power corrupts. We've seen it many times in the Old Testament; One particular example of this (off the top of my head) is David, who was a prophet, and a very strong spiritual leader at that.
So, for the actual question:
Are we to understand that Jesus taught separation of Church and state as part of His new Gospel? If not, are we to understand that unification of the Church and the state government is a desirable thing (like in some Islamic states)?
We have, of course, seen the Catholic Church act as (or have a strong influence on) the government in pre-Protestant times; as we all know, the Catholic Church dealt with a lot of corruption in the leadership, so I don't think this can be taken as evidence of a formal declaration of doctrine.