I'm not sure I have learned yet how to find the root of a sin, but this is an excellent passage that makes clear that to truly mortify a sin you must strike at the root.
A man may beat down the bitter fruit from an evil tree until he is weary; whilst the root abides in strength and vigor, the beating down of the present fruit will not hinder it from bringing forth more. This is the folly of some men; they set themselves with all earnestness and diligence against the appearing eruption of lust, but, leaving the principle and root untouched, perhaps unsearched out, they make but little progress in this work of mortification.
How does one find the root of a sin? It is important to know that this is a question that must be answered.
It reminds me of a theme that runs through the book of the comprehensiveness of our assault on sin. Sin is too strong to be defeated by an effort here and there, or even by an effort for a season. Sin must be approached not as a single battle, but as a war. Sin is a large thing.
We should build strong defenses against sin and launch coordinated attacks on it. When we have the advantage, we should press it, not be satisfied with one victory. And this war will never end by our own doing. Jesus can end it by returning, but unless he does, we will struggle against sin our entire life. It is a marathon, not a sprint.
Even when it seems like we have conquered sin, and its armies are scattered and weak, we should be watchful and on our guard. Perhaps it is a trap. Or perhaps it really is defeated, yet all it needs is neglect to begin to gather strength again.
Our strongest attack against sin is to grow in our love of God through the Spirit. When we rejoice in God and find satisfaction and peace in knowing him, this is deadly to sin, and it can cut down even the strongest temptation and the most clever lie. To embrace the Spirit is to reject the flesh.
But I say, walk by the Spirit, and you will not gratify the desires of the flesh. For the desires of the flesh are against the Spirit, and the desires of the Spirit are against the flesh, for these are opposed to each other, to keep you from doing the things you want to do.