It's a tricky one.

Russell Moore, president of the Southern Baptist Convention's Ethics & Religious Liberty Commission and not by any means the most hard-line of that ilk, has come down hard on pastors who take the weddings of people who are living together without being married or who aren't Christians.

According to the Christian Post, speaking at a Church and Sexuality conference in Montgomery, Alabama, Moore said: "You cannot marry anyone except believers and people under the authority of Jesus Christ" because you "cannot hold [unbelievers] accountable to their vows".

And if a deacon wants his pastor to conduct the wedding of his daughter who is 'living in sin', he should say no. "It takes courage not to do weddings, to say, 'I'm not going to do that,'" Moore said.

The issue is one that defines not just attitudes toward sexual morality, though it does do that. More fundamentally, it says something about the nature of the Church and how the Church relates to the world.

Is it it a tight-knit community of the redeemed, a body of believers who are signed up to the same cause and who are marching in step to the Celestial City? Or can it have porous boundaries? Should it be acknowledging that no one's perfect and celebrating their spiritual progress as they move from shacking up together to the sacred commitment of marriage?

If you're an Anglican in the UK, you're signed up to the latter understanding by default. The CofE's Your Church Wedding website says: "It is both a privilege and a duty for your vicar to join a man and a woman in marriage. The good news is that you can marry in church regardless of your beliefs, whether or not you are christened and whether or not you have been a regular churchgoer."

This is because the Church of England takes the line that it's there to serve the whole community. Marriage, like baptisms and funerals, are services it offers as part of its mission. It sees them as opportunities to mediate the grace of God, to make pastoral contact with people and to have meaningful conversations about faith and the deep things of life.

But a relaxed attitude toward personal faith throws up, as Moore rightly says, all sorts of theological problems. While Anglicans face this issue more than most denominations because, generally speaking, their buildings are prettier and more people want to get married there, other Churches face it too.

In fact, there are problems with both 'open' and 'closed' positions.

If a church or its pastor decides it will only marry Christians whose behaviour is morally upright and who are marrying other Christians, it's acting as a 'gatekeeper' to the sacrament. All the words said and vows taken will be by people who believe them. There is a depth of meaning to the promises that are made which would not be there otherwise. It's marriage within the 'tribe'; there is nothing disruptive to the life of the community about it.

But this attitude doesn't acknowledge the complexity of human relationships. It doesn't seize the pastoral opportunities offered by a more open model. It runs into inconsistency – if a Christian couple are living together and can't get married in church but attend faithfully after they get married elsewhere, it's hard to know exactly what status they have. Should they be allowed to sing in the choir? Is the church happy to take their offerings?

If a church takes a more open position, it has the ability to minister to people at a significant moment in their lives. It can come alongside them in a pastoral and evangelistic way. It shows a loving and welcoming spirit.

But there are problems with this, too. Is it right to lead people in making promises about their marriage to a God in whom they may not believe? Is a certain standard of sexual conduct to be expected of someone before their relationship can be blessed in a marriage service? How comfortable is the church about being used because it's pretty, in the same way that a couple might choose a nice venue for their reception?

So the marriage issue encapsulates much about how the Church relates to the world. In fact the either/or description suggested earlier doesn't quite work. It's both a tight-knit community of believers, the body of Christ on earth, and a community with open arms and a warm heart, reaching out to sinners and unbelievers everywhere.

The real question is how to act with integrity and still demonstrate the loving welcome of the Lord Jesus himself. It's hard to believe that Jesus wouldn't have been glad to see two people making a marriage commitment to each other, whatever their past. But it's hard, too, to believe it would be right to encourage people to say words they don't believe. There's a place there for serious conversations about why someone wants to get married in church – and sometimes the answer ought to be "no".