Verse 12. - For it is a fire that consumeth to destruction; i.e. it is a thing which brings down the wrath of God upon a man, so that "a fire is kindled in his anger, which shall burn unto the lowest hell" (Deuteronomy 32:22). Compare the sentence on David for his great transgression (2 Samuel 12:9-12). And would root out all mine increase; i.e. "would destroy all my estate;" either by leading me to waste my substance upon my companion in sin, or by bringing down God's judgments upon me to my temporal ruin.

31:9-15 All the defilements of the life come from a deceived heart. Lust is a fire in the soul: those that indulge it, are said to burn. It consumes all that is good there, and lays the conscience waste. It kindles the fire of God's wrath, which, if not quenched by the blood of Christ, will consume even to eternal destruction. It consumes the body; it consumes the substance. Burning lusts bring burning judgments. Job had a numerous household, and he managed it well. He considered that he had a Master in heaven; and as we are undone if God should be severe with us, we ought to be mild and gentle towards all with whom we have to do.For it is a fire that consumeth to destruction,.... Referring either to the nature of the sin of uncleanness; it is inflammatory, a burning lust, a fire burning in the breast; see 1 Corinthians 7:9; or to the effect of it, either the rage of jealousy in the injured person, which is exceeding fierce, furious, and cruel, like devouring fire, not to be appeased or mitigated, Proverbs 6:34; or else it may respect the punishment of this sin in the times of Job, and which we find was practised among the Gentiles, as the Canaanites, Job's neighbours, burning such delinquents with fire; see Genesis 38:24; or rather the wrath of God for it, which is poured forth as fire, and burns to the lowest hell, and into which lake of fire all such impure persons will be cast, unless the grace of God prevents; and which will be a fire that will consume and destroy both soul and body, and so be an utter and everlasting destruction, Revelation 21:8;

and would root out all my increase; even in this world; adultery is a sin that not only ruins a man's character, fixes an indelible blot upon him, a reproach that shall not be wiped off, and consumes a man's body, and destroys the health of it, but his substance also, the increase of his fields, and of his fruits, and by means of it a man is brought to a piece of bread, to beg it, and to be glad of it, Proverbs 6:26.