In 1791 Baptist minister and leader John Leland said, "It is not supposable that any established creed contains the whole truth and nothing but truth; but supposing it did, which established church has got it?" He was underscoring one of the many problems in uniting church and state. If Christian principles are to be promoted by the government, whose version of Christianity is to be advanced? Catholic or Protestant? (Think Northern Ireland here.) If we pick a denomination, which one should we choose? What will we do about the doctrinal differences within the ranks of that denomination?
Would we try to develop a governmental form of Christianity that resolves all the differing views within Christendom? Does anyone think that would ever work? Does anyone like the idea of a government approved gospel?
Better to let the church advance the cause of Christ and leave government out of the effort. After all, when Satan offered Jesus the governments of the world, the Lord turned him down.