The question of whether God’s grace provides licence to sin has been the subject of intense debate ever since Paul wrote his letter to the Roman church. Here’s something I read about it yesterday:

The main who sins, presuming grace will abound, self-assured that God must see him through, can hear no word of grace. His state is one where forgiveness is impossible. The Everlasting Mercy can only come to those who have wholly ceased to take it for granted. The Word that speaks redemption can come only as a ‘nevertheless’, that is, to those who recognize that, as far as they have power to evade it or right to reckon, nothing can alter their state of eternal loss.

— John A. T. Robinson, In the End God

In other words, before you can truly receive grace, you have to have reached the end of your rope. It’s like the alcoholic who has finally reached rock bottom: only from that place can he finally admit that he is powerless to rescue himself.