Because it is invariably brought up in any discussion about God and Mammon, let's clarify one point.

Jesus did not throw the money-lenders out of the Temple, not least because there weren't any money-lenders in the Temple.

The incident usually known as the Cleansing of the Temple is mentioned in all four Gospels (Matthew 21; Mark 11; Luke 19; and John 2, if you fancy looking it up). They all agree that the people whom Jesus chucked out when he "whummelt the tables", as Lorimer's excellent Scots translation of St Mark puts it, were money-changers (and pigeon sellers).

They couldn't have been money-lenders because Jews were not allowed to lend money at interest to other Jews (though they could to Gentiles). Greek and Roman coins were not acceptable for Temple donations, so there was a sort of bureau de change for turning them into Jewish money. The distinction is important, because traditional Jewish, Christian and Islamic notions of usury distinguish between commerce and lending money at interest.

Money-changing was a commercial operation, not a usurious one, which makes Christ's action, if anything, even more radical. It is also the only instance in the New Testament when Jesus uses physical force.

Money-lending has always been problematic for religious groups, as the Archbishop of Canterbury discovered last week when launching a crusade against the payday loan company Wonga, only to discover that the Church of England had invested in the business. Although it was very indirectly, and a very small sum compared with its overall investments, he immediately conceded that this was embarrassing. (Politicians might try this little-used tactic, known as telling the truth.)

The Church of Scotland, which has already said it would like to follow the archbishop's initiative, is sensibly first checking its own investments to ensure that it doesn't have any money in similar enterprises.

The condemnation of lending money at interest is by no means as straightforward as it might seem; the Sermon on the Mount declares that there is a moral duty to lend money, while in the parable of the talents (which in Luke is actually in the same chapter as the Cleansing of the Temple), the feckless servant is upbraided for having failed to put his money out on the exchanges to get a return.

But one of the chief obstacles the poor face is being unable to borrow money, which is why micro-lending schemes for Third World countries are one of the most effective forms of aid. In practice, Christians have always been allowed to charge some interest on loans; the Council of Nicea, for example, ruled that interest should be capped at 12.7% APR, though it didn't put it in quite those terms.

The chief justifications given were usually that the lender should either share in the venture being financed, and compensated for the risk, or that he should gain some allowance for the loss of profit which he might have made had he invested the money himself – a doctrine very similar to the economic notion of opportunity cost.

Islam, which has historically taken a stricter view of lending at interest, has nonetheless created a successful banking system which builds on similar ideas, such as risk-sharing and fees for safekeeping.

But no matter how generously or loosely one defines usury, I think we can be fairly confident that the advertised APR of Wonga, which is a staggering 5853%, would be judged by most theologians – not to mention anyone of any faith or none who has half an eye and the ability to count to three – to be outrageously usurious.

This doesn't seem to bother the firm, which, to be fair, makes no pretence about its charges and points out that it is not designed for long-term lending (its current maximum term is 46 days).

Wonga acknowledges that it is not the cheapest or most sensible way to borrow money, but believes it provides a convenient and efficient service for those who require loans for very short periods, and may find them difficult to get from more conventional lenders.

The problem for payday lenders of this sort, however, is that their customers are overwhelmingly drawn from the poorest sections of society. Of course Wonga and firms like it are operating within the law, and people are free to choose their service or not. But the reality for many poor people is that they do not really have that element of choice.

The refreshing thing about Justin Welby's intervention in this debate is that he did not choose to call for lenders with high interest rates to be outlawed – indeed, he has acknowledged that they can be well run and are infinitely better than illegal street lenders who are likely to collect their payments with the aid of a claw hammer. Instead, he has suggested using churches to help expand the reach of credit unions, something which the Kirk is also considering.

By declaring he would like to "compete Wonga out of business", the archbishop has played a blinder, and won approval from all points in the political spectrum. The right likes it, because it's entirely consistent with free markets. The traditional left approves of mutualism, and better rates for the poor (though a credit union's maximum 2% a month is still, in fact, twice the Council of Nicea's upper limit). Even Wonga has said that it welcomes competition as good for customers – something which businesses always pretend to approve of, even if most of them secretly like markets only when they work for their benefit.

The archbishop's background in commerce, as well as his acknowledgement that the Church, if it is to operate in the secular world, is bound to be imperfect, gives weight to his plans. But their primary usefulness is not merely to help those who find it difficult to borrow small sums at reasonable rates – valuable though that is. It is also a declaration that the Church can, and should, be actively involved in helping the most vulnerable, and that it should be taking its place in the public square.

The Church of Scotland's General Assembly may have called last year for legislation to cap interest rates, but the Kirk's decision to back Archbishop Welby's scheme – as well as its announcement that it is considering funding credit unions, is a more effective and concrete demonstration of commitment. Like food banks, many of which operate from church networks, facilitating credit unions is a practical manifestation of the Christian imperative to help those in need.

There's nothing particularly virtuous about trying to outlaw businesses which you think morally objectionable; providing a real alternative, however, is not only a spiritual, but a material, good.