Rom. 8:5-6 Those who are dominated by the sinful nature think about sinful things, but those who are controlled by the Holy Spirit think about things that please the Spirit. If your sinful nature controls your mind, there is death. But if the Holy Spirit controls your mind, there is life and peace.
What do you think about the most? Whatever it is, it's a good indicator of where you are at spiritually. If you are always thinking about things that are not holy, then your heart is not right with God and you are heading for certain spiritual death. If your thoughts are more geared to godly things, then you are more apt to be right with God.
Now, I know that it's not possible to always avoid sinful thoughts and always focus on godly things. We life in the flesh in a very sinful world and our minds do pick up on our surroundings and sinful thoughts do come. However, if your soul is healthy, you will not dwell on those things. You will redirect your thoughts away from those things so that you can be more focused on godly things.
When we make the conscious choice to dismiss evil thoughts, we discover that there is reassurance of our salvation and peace with God. Only the Spirit of Christ can set us free from the bondage of sinful thoughts. He pulls us away from them so that they do not become fixtures in our minds. We gain victory over their hold on us when we let go of them and let the Spirit have His place in us instead.
If you are struggling and can't seem to enjoy peace in your relationship with God, check your thought life. Do not linger on those things which are sinful. Do not let yourself be entertained by them. Do not stay around where those things prevail. Protect your mind and you are protecting your soul.