The reason why true repentance is to examine not only the actions of our life but also the intentions of our will is that our understanding and our will produce our actions. We speak from our thought and we act from our will; therefore our speech is our thought speaking, and our action is our will acting. Since this is the origin of what we say and do, it is clear without a doubt that it is these two faculties that commit the sin when our body sins.
It is in fact possible for us to repent of evil things we have done through our bodies but still think about evil and will it. This is like cutting down the trunk of a bad type of tree but leaving its root still in the ground; the same bad tree grows up from the root again and also spreads itself around. There is a different outcome when the root is pulled up, though; and this is what happens within us when we explore the intentions of our will and lay our evils aside through repentance.
We explore the intentions of our will by exploring our thoughts. Our intentions reveal themselves in our thoughts—for example, when we contemplate, will, and intend acts of revenge, adultery, theft, or false witness, or entertain desires for those things. This applies as well to acts of blasphemy against God, against the holy Word, and against the church, and so on.
If we keep our minds focused on these issues, and explore whether we would do such things if no fear of the law or concern for our reputation stood in the way, and if after this exploration we decide that we do not will those things, because they are sins, then we are practicing a repentance that is true and deep. This is even more the case when we are feeling delight in those evils and are free to do them, but at that moment we resist and abstain. If we practice this over and over, then when our evils come back we sense our delight in them as something unpleasant, and in time we condemn them to hell. This is the meaning of these words of the Lord: “Any who try to find their soul will lose it, and any who lose their soul for my sake, will find it” (Matthew 10:39)
From Regeneration, Pages 33, 34