Any number of counter-questions might be appropriate:
Is fear of punishment the only reason we can imagine for moral choices?
Isn't unconditional love most likely to elicit good living?
Why is being good the ultimate arbiter here (rather than the question of my relationship to God)?
But as we spoke it seemed clear to me that the big misconception behind it all was a view that says: The Christian life is really, really, really hard and the only reason to live it is because there are other, basically unrelated, spiritual rewards. Take away these carrots and sticks and of course you'll sin. Because, you know, sin is really great. It's so great that God has to threaten us with hell to stop us having fun. Offer free grace and there'll be pandemonium.
As though the way of Jesus is stifling.
As though sin is life-giving.
As though God's a cosmic kill-joy.
As though only eternal damnation balances the scales enough to make Christianity the clever choice.
As though the Christian life is so horrible, we need harsh sanctions to remain in it.
As though Jesus said "Toughen up people. My yoke is hard, but hell is harder."
But what if knowing Christ Himself is the centre of the Christian life rather than sin management? And what if Jesus really brings life and sin only brings death? What if Christ's yoke really is the easy one - the only one that properly fits? What if being good is just really, you know, good? And what if you don't have to dangle people's feet over the pit to get them to behave? What then?
Well you tell the believer "You're in Christ, securely and unconditionally, so why be bad?"