If you know your Bible at all there is a myriad of stories of faithless people who turned out useful to God and His plans. The one I love the most out of the Old Testament is Samson. God said Samson would destroy the Philistines. But it looked like Samson was going to make love and not war. In short, God got His way through Samson trying to get his way. It is better if you don't sin, but if you do, that does not mean your life is not still under His rule. He will get His way.