"Render to Caesar what is Caesar's and to God what is God's." (Jesus Christ)
Jesus wasn't about religion, in fact he reserved his anger for organised religion and challenged, ignored or subverted religious practices. He didn't teach the non-religious about religion, or expect them to become 'religious'. If anyone has to become religious to follow Jesus they've missed the point, so talk of the relationship between church and state is missing the point too. It really is debateable whether Jesus wanted a Church. In fact, Jesus was a humanist. We should be humanists too. If Jesus' followers fail to adopt this outlook they revert to being 'religious' instead, if this then leads to people becoming out of tune with humanity they may go on to becoming inhumane. There are too many examples of this happening in the history of Christianity, pray God that it does not continue to happen. However, we see 'religious' inhumanity in appalling 'Christian' attitudes towards Muslims - 'you have heard it said that other religions are false, give their followers the cold shoulder and make them feel unwelcome.' Jesus never said this but you might think that he did when you hear what some of his followers say.
Jesus actually advocates humanism - 'give to man what is man's', or better, 'you have heard it said, "it's a man's world", but I say to you, "give to woman what is woman's" - what possibilities does this offer? Is this what religion looks like? Hopefully it is, though religion has often treated women incredibly unfairly, giving or expecting legal permission to treat them unequally, and continues to do so. What if Jesus was saying - 'Give to human beings their human rights and entitlements, by the time you've done that you will have given God what God wants?' This is what God wants actually, God invented human rights! This ensures that religion doesn't get in the way of people. Religion asks - 'how should I treat my neighbour?' This is a secular question. True religion asks and answers secular questions, questions for everybody, not religious questions with religious answers for religious people.
We should turn to secular scenarios to test our religion properly. Secularism is a good test for religion. If we can allow that Jesus was not religious and urged us not to get stuck in religion, then we should be open to the secular, everything that is not explicitly religious, and be comfortable with it. The challenge for 'believers', which is 'loving our neighbour as ourself', is to live life and faith in the secular sphere. This isn't what those who questioned Jesus wanted, their twin aims were to trick him and twist out of anything that they thought he might say. In his responses he generally offered a humanistic answer which trumped 'religion' - for example, when questioned about 'working' on the Sabbath- 'who among you will not rescue his donkey from a ditch?' He also offered humanistic observations in his sermons, reflecting on God's care for outsiders and foreigners.
His 'give to Caesar' is really asking us a big question - does commitment to God cease when you are part of wider society? If you don't pay your taxes to Caesar then you are limiting and excluding God who expects you to do so. God does indeed support the secular establishment.
In a society where religious people are a 'special case' (increasingly a minority), it may be more difficult to make sense of this, which means we should try harder. Nonconformists need no persuading that there should not be a religious Establishment, though inevitably we have grown accustomed and accommodating towards it. Even in a 'Christian' country with an Established Church there is an accommodation and interdependence between the religious and the secular. The whole of society - sisters and brothers, neighbours - should have equal regard for each other. This is tested and stretched on sensitive issues, like same sex marriage, where there is more of the Gospel in secularity and humanism in society than in the church.
If the churches are permitted to exclude equal marriage, because Caesar's writ does not run in the Divine world of 'religious' buildings ... then we have either tricked Caesar into allowing this or given God privilege over the Emperor's right, God claims and expects no privilege ... God is comfortable with the secular too.
At the very least, religious people should 'rejoice with those who rejoice' (St Paul in Romans 12.15) who are having same sex marriages outside the church, though this does not seem to have happened in any significant way, except at the personal level. Religious people should be wholeheartedly supportive and appreciative of the secular as it is there for everybody, perhaps even embodying Gospel values that Christians may have forgotten about - Caesar helps us to reflect on and learn about God, something else that Jesus was trying to teach us.
We should be finding ways to show that we are comfortably secular - rather than uncomfortably religious. Are religious people joining in the conversations that everybody else is having? If the religious are too uncomfortably religious to be comfortably secular then a religious problem is created but the secular gets the blame. Religion should not be able to escape equality there is no religious get out in the face of the questions that everyone else is asking, only escapism - 'No we're not going to engage constructively and humanely with same sex marriage'. Yes, you've guessed it, this makes religious people irrelevant and Caesar is not given his due, and we end up on the wrong side of Jesus.
The religious should also be in a conversation with Caesar. We have a right, a non-religious one, to both question the authorities and to be listened to by them. Perhaps this is where we can remind them of the humane, the humanistic and the secular, when they talk about a Christian country but mistreat the poor. Maybe Christians can only truly serve God and live the Gospel by rejecting a religious label for our society in favour of something radically humanist and tell Caesar that he (it's almost always a he), should respect his limitations and responsibilities. God isn't interested in competing with Caesar or labelling or causing tensions, nor in carving out a privileged space or demonising. God is deeper, and more humanistic. As religious people we should be taking our God-consciousness and using this to focus on people and our place in the world, because there isn't anything else. Caesar has much to teach us about the Gospel ...