I think we, as Christians, often forget how big God is, how great and amazing His works are, how wondrous the things He performs in people's lives. It really bothers me some times when I hear preachers preach that if you do this, or this, God will not be able to use you, you've ruint your life and that's that. God is so much greater than that! Often, I think they forget where God brought them from, and that He is still able to use them. I'm not condoning sin, or saying that you can live a wicked life and get away with it as if nothing ever happened. But He is the God of forgiveness. I believe we do things that sometimes change the way God can use us. Because of bad choices we make, we can no longer be used in some of the most obvious ways. Sometimes, God uses us in ways only He can see, ways that are only known well after the fact. God can use anyone, anytime, anywhere, any way He chooses! After all, He is God, and not controlled by our thinking.
This morning, Bro. Mike Bagwell has a wonderful lesson on this subject on his website. (Lesson 3: Ruth) Go there and read it. It might just change some of your thinking.