For many evangelical Christians, it's a given: you tithe your income. A tenth of what you earn – pre-tax or after tax, the jury's out on that one – is to be given away, probably to your church.

It's a teaching that's extremely useful to pastors and church leaders, because it means their churches' ministries can be well funded. On the other hand, a South Korean theologian, Ahn Yong-su, has argued in a recent book that the whole practice is unbiblical and potentially abusive.

Reported by Korean news outlet the Hankyoreh, Yong-su, author of The Tithe Nailed To The Cross, describes churchgoers as "slaves to the tithe".

"Pastors have treated tithing as a standard for judging people's faith," he says. "You need faith to receive the salvation that opens the door to heaven, and pastors say that people with faith must tithe. In other words, you've got to tithe if you want to go to heaven. They've created a link between faith, salvation and tithing."

And, he says: "Pastors also say that people who don't tithe are shamelessly stealing from God. The tithe is seen as representing your loyalty to God... Poor people who can't tithe much or at all are treated like unbelievers."

Ahn Yong-su is highly critical of particular practices in some South Korean churches that most readers would immediately recognise as abusive. No one could justify that sort of behaviour. And in UK churches, teaching about tithing is likely to be more low-key; we're just less comfortable talking about money than other cultures.

But look harder, and it's hard to justify teaching tithing as a Christian duty at all.

In the Old Testament, tithes were a requirement of the law. The Israelites were to give 10 per cent of their crops and livestock (Leviticus 27:30-34). In Numbers 18:21 the tithe was applied to the maintenance of the Levites, who owned no land but served as priests.

But the tithe was not the only offering the Israelites were to make. Deuteronomy 12:5-6 talks about "your burnt offerings and your sacrifices, your tithes and the contribution that you present, your vow offerings, your freewill offerings, and the firstborn of your herd and of your flock".

And tithing was taken very seriously. The prophet Malachi castigates the people for not bringing in "the full tithe to the storehouse" and "robbing" God (3:8-12). If they bring their tithe, he says, God will "open the windows of heaven for you and pour down for you a blessing until there is no more need" – in other words, if they give to God, he will give to them.

The trouble is that it's very easy to take verses like these and apply them uncritically to our own times without looking at what the New Testament says. And it's clear in the New Testament that things are going to be different. Jesus moves the emphasis away from the mechanical act of giving to the motivation behind it: "Woe to you, scribes and Pharisees, hypocrites! For you tithe mint and dill and cumin, and have neglected the weightier matters of the law: justice and mercy and faithfulness. These you ought to have done, without neglecting the others," he says (Matthew 23:23-24). In 2 Corinthians 9:7, Paul says: "Each one must give as he has decided in his heart, not reluctantly or under compulsion, for God loves a cheerful giver." Furthermore, in Acts 2:44-45 we're told, "And all who believed were together and had all things in common. And they were selling their possessions and belongings and distributing the proceeds to all, as any had need."

So while Jesus tells the Pharisees to carry on tithing – because it is commanded for Jews – it looks as though the tithe is nowhere in the minds of the early Christians. How much they are to give is left to their own consciences; it isn't laid down anywhere in the New Testament. 

And we can see why this might be. Teaching people to tithe is potentially spiritually dangerous, for three reasons.

1. It limits generosity.

The danger is that it implies that once someone has given a tenth of their income, they've done all they need to do. It is not a way of encouraging giving, but limiting it. Instead of being an offering the tithe is a tax, as it was in the Old Testament, and this runs counter to the whole spirit of the gospel. We believe in freedom, not legalism.

2. It's potentially abusive.

Tithing is taught in evangelical churches partly because many of them don't have historic endowments (like the Church of England) and rely almost entirely on offerings. This means there is a temptation to try to maximise income by stressing the importance of giving. When this isn't controlled by rigorous biblical and ethical principles, it can be open to abuse, as Ahn Yong-su found. It's wrong to say that Christians who don't tithe are second-class believers.

3. People might not be able to afford it.

You might not miss a tenth of your income if you have a lot of money. Or even if you do miss it, you can just get used to living on less. But some people's income is so low that it would cause them serious problems if they were to give away a tenth of it. We should not put people's financial stability in danger through misguided teaching, but trust them to do the right thing.

Some people use the verses from Malachi about God "opening windows from heaven" if the people tithe to argue that he will never let people get into financial difficulty because of their offerings to him. But we should be very careful before we take a specific passage addressed to Israel and make it a general rule for every believer today.

Behind these negatives, though, there are some important principles. Christians should be generous. We shouldn't seek money for ourselves and we should hold it lightly, always being willing to give it away. Giving should be free. It's not up to anyone else to tell us how much to give, and if anyone tries to do so they have gone beyond what the Bible allows. And giving should be worshipful. When we give – whether it's a tithe, or more, or less – we are giving to the work of God, in the sight of God.

Money and possessions are gifts from God. But unless we can give them away, they become a trap. Churches that teach tithing as a duty are running serious spiritual risks with their congregations.

Follow Mark Woods on Twitter: @RevMarkWoods