It was your standard after church, five sandwiches and two chicken wings luncheon. I had enjoyed preaching and worshipping with the sisters and brothers in a wonderful church in the heart of darkest Englandshire, and gratefully accepted the invitation to stay on for the after service grub. As I sat beside a normal looking middle-aged couple I couldn't help but notice the look of sympathy that passers by gave. After the usual introductions I soon found out why. "We really appreciated your sermon, David, in fact we feel that you are the kind of open-minded person who would be open to joining our movement". "What movement is that?" "The Christian naturists club". I managed a polite refusal but could not help but wonder why they picked on me!

In case you happen to be wondering there are two reasons I won't be joining up soon – one, you should see my body (actually you shouldn't!), and two I am still a sinner living in a sinful world – to pretend to be innocently naked in paradise is as sinful as it is silly.

I was reminded of this in reading some recent articles on Christian Today. The first was from a couple of 'Christian' swingers who have sex for Jesus. In what at first seemed a complete parody (but sadly wasn't), two bodybuilders from Florida who were described as 'a devout Christian couple', justified their sexual promiscuity by declaring it a form of evangelism. "You can't get closer to someone than having sex with them"! (incidentally can I note in passing what a shallow and superficial view of intimacy!). They reminded me of a cult which in the 1970s used to practice what they called 'flirty fishing'.

And then there was the story of Farah Williams, a 23 year 'erotic writer' and porn star, who says that faith is "number one in my heart and mind" and that "No matter where I am, whether it's in a gentlemen's club or church, I know my faith is what guides me,". She added that she 'lets God lead her life and faith, enough said".

Or did you hear the one about the militant racist who likes to organize violent attacks upon black people, but feels that "my faith is my number one and if I can lead my fellow racists to faith in Christ, then I feel I am doing what the Lord wants. While I admit it looks strange I don't think that other people should judge me with their absolutist standards. God is a 50 shades of grey loving God".

Sadly only the last of these examples is made up. Apparently the first two are real. What is even more astonishing is that there are Christians who are so biblically illiterate, emotionally bullied and culturally ignorant that they actually buy into these stories. Although extreme this is what happens when you prostitute faith. We use the term 'prostitute' (although that will soon be illegal and we will be compelled to use the 'sex worker') to describe someone who sells themselves for sex.

I use it in a spiritual sense to describe those who sell their faith (and their saviour) just to justify their own sinful behaviour. Having sex with strangers, or providing sexually erotic material for voyeurs, is not an act of Christian witness or a demonstration of faith. God has not guided you that way, any more than he has guided the lunatic who thinks he has been told to kill prostitutes, or abuse unbelievers. It is an example of a me-centred, self-obsessed, perverted religion that is as far from Christ as it is from humanity. It is not the act of a 'devout' Christian couple; it is the doctrine of demons. To be honest I prefer 'honest' prostitutes to such dishonest Christians with their fake faith and nonsensical spiritual talk. At least the former are far more likely to recognise their sin and seek the wholeness and forgiveness that comes through Christ (as long as they don't meet the latter first of all!).

I suspect that the vast majority of Christians will agree with this assessment and would only read these stories as a kind of spiritual porn, watching in horror as we see the extent to which the prostitution of the Christian faith can go. But the trouble is that this is the inevitable form of a much milder perversion of faith. The kind of thing that influential pastors like Craig Gross argue for when they declare that the theology around homosexuality for example is not black and white. They ask us to believe in a grey God who is more interested in people than doctrine or absolutist ethics. It all sounds so nice and reasonable. But it is actually false and irrational. Do you think that they would say the same about racism, greed or violence? They have a pick 'n' mix attitude to the Bible which is more concerned about what the culture thinks, than the holiness of God.

Of course it is true that we have no right to impose our doctrines and our ethics on to God, but we have even less right to reject the doctrines and standards of God, and just invent our own personal religion and Jesus. What some evangelicals seem to have bought into, in order to maintain their status, credibility and finance in contemporary society, is a Jesus of their own making. Here is a challenge for the Church today. Instead of blurring the lines, where Scripture does not, we need to make a more distinctive stand. The old Baptist preacher, CH Spurgeon was surely right when he declared "The problem is that people bear the Christian name but act like worldlings and love the amusements and follies of the world. It is time for a division in the house of the Lord in which those for Christ go into one camp and those against Christ go into the other camp. We have been mixed together too long."

David Robertson leads St Peter's Dundee and is the director of Solas CPC