Do you need God to be good? Not according to famed Christian-baiter Richard Dawkins.

He said in an interview with Gay Byrne on Ireland's RTE One on Sunday: "If you're good because you want to curry favour with God, if you're good because you want to avoid going to Hell, or if you want to go to Heaven, that's a rather ignoble, self-centred reason to be good."

It's rare that a Christian finds himself in complete agreement with Dawkins, but in this case I think he's absolutely right. Christians try to be good not because we are afraid of what God will do to us if we aren't, or even because we want to get to heaven, but out of a whole-hearted, loving response to the love that God has shown to us.

At least, that's what we tell ourselves. But in reality there's more to it than that. It's complicated, as Dawkins implies, by the doctrine of eternal punishment, which hovers over our evangelistic efforts as a bird of prey hovers over a field vole.

Underlying what Dawkins says is his perception of a paradox. Christians genuinely want to love God and do the right thing. But if we're brought to do that by the knowledge that there are serious and eternal consequences if we don't, we aren't behaving morally at all; it's just pragmatism.

If I don't sleep with my neighbour's wife because I think it's wrong to do so, I'm behaving morally.

If I don't do it because I'm scared I'll be found out, I'm not being moral, any more than I'm being moral if I look both ways before crossing a road. I'm just being sensible.

There's a Facebook meme going around at the moment using that picture of Jesus knocking on the door of someone's heart. The dialogue goes: "Let me in!" "Why?" "So I can save you." "From what?" "From what I'm going to do you if you don't let me in."

Like many such memes, it's effective because it takes a warm, cosy image and subverts it.

We say that evangelism is good news. We want people to become Christians because they're convinced that God loves them and wants the best for them. But can we honestly make that appeal to them while still holding in reserve the doctrine of eternal punishment?

Christians have always been aware of this paradox. John Calvin, no less – who could never be accused of softy liberalism – wrote in his Institutes (Iii2) of the Christian mind that "restrains itself from sinning, not out of dread of punishment alone; but because it loves and reveres God as Father, it worships and adores him as Lord. Even if there were no hell, it would still shudder at offending him alone."

In days gone by, evangelists used to be far more up front about hell. The 18th century preacher Jonathan Edwards gave a famous sermon entitled The Eternity of Hell Torments in which he asked his hearers to "consider what it is to suffer extreme torment forever and ever: to suffer it day and night from one year to another, from one age to another, and from one thousand ages to another... in pain, in wailing and lamenting, groaning and shrieking, and gnashing your teeth – with your souls full of dreadful grief and amazement, [and] with your bodies and every member full of racking torture; without any possibility of getting ease; without any possibility of moving God to pity by your cries; without any possibility of hiding yourselves from him; without any possibility of diverting your thoughts from your pain; without any possibility of obtaining any manner of mitigation, or help, or change for the better".

We don't very often hear such sermons now. But the doctrine itself is part of our evangelical furniture, and any attempt to question it – by Rob Bell, in his book Love Wins, for instance – draws down instant repudiation (John Piper, the hugely influential Calvinist theologian and pastor, notoriously sent a three-word tweet: "Farewell, Rob Bell" when the book came out).

In fact there are strong biblical arguments against the doctrine, as 'Gregory MacDonald' (it's a pseudonym) shows in The Evangelical Universalist: The Biblical Hope that God's Love Will Save Us All. At the very least, it shows that the issue isn't quite as cut and dried as evangelicals usually think it is.

In the meantime, we're left with the paradox: we want to love God for God's own sake and to serve him because we've been given a vision of his own goodness and we want to share it. But the threat of what will happen if we don't believe is always lurking under the surface – and as Dawkins says, it's not a very worthy motivation.

So what should Christians do? One answer is to ditch the idea of God's judgment altogether. That may be a step too far for most evangelicals. Even Robin Parry (MacDonald's real name) writes: "To jettison such a long-standing and clear tradition is something that should be done cautiously and reluctantly." However we understand it, the idea that God is a God of justice and that the wicked will face judgment seems to be built in to scripture – and it seems to be a universal human desire, too.

In the end, the paradox is insoluble. An active belief in eternal punishment will always compromise a loving response to God's loving outreach to us. A mature faith moves towards thankfulness for God's goodness and trust in his grace, rather than fear of his wrath.

But a better approach to evangelism, it seems to me, is not to threaten people into the Kingdom, but to love them into it. The fundamental truth that we have to tell is this: "The Word became flesh and made his dwelling among us. We have seen his glory, the glory of the one and only, who came from the Father, full of grace and truth" (John 1: 14). Life with Christ is simply better, richer and fuller than life without him, and the world needs what he has to give to it.

Follow @RevMarkWoods on Twitter.