Unbelief is the root of many sin patterns in our life. It often lies under many of the mental, spiritual and emotional issues we face in life. So what is it that we don't believe? Typically it is God's hard truths - which may be inconvenient to believe.
Another root of sin is selfishness. In fact, look at every type of sin and one will usually find some dimension of selfishness or self-centeredness.
So if I want to address the sin in my life, I might want to start by reconciling my thinking (beliefs) with God's truths and by investigating the level of selfishness that reigns in me.