Then when lust hath conceived. He first calls that lust which is not any kind of evil affection or desire, but that which is the fountain of all evil affections; by which, as he shews, are conceived vicious broods, which at length break forth into sins. It seems, however, improper, and not according to the usage of Scripture, to restrict the word sin to outward works, as though indeed lust itself were not a sin, and as though corrupt desires, remaining closed up within and suppressed, were not so many sins. But as the use of a word is various, there is nothing unreasonable if it be taken here, as in many other places, for actual sin. And the Papists ignorantly lay hold on this passage, and seek to prove from it that vicious, yea, filthy, wicked, and the most abominable lusts are not sins, provided there is no assent; for James does not shew when sin begins to be born, so as to be sin, and so accounted by God, but when it breaks forth. For he proceeds gradually and shews that the consummation of sin is eternal death, and that sin arises from depraved desires, and that these depraved desires or affections have their root in lust. It hence follows that men gather fruit in eternal perdition, and fruit which they have procured for themselves. By perfected sin, therefore, I understand, not any one act of sin perpetrated, but the completed course of sinning. For though death is merited by every sin whatever, yet it is said to be the reward of an ungodly and wicked life. Hence is the dotage of those confuted who conclude from these words, that sin is not mortal until it breaks forth, as they say, into an external act. Nor is this what James treats of; but his object was only this, to teach that there is in us the root of our own destruction.

Then when lust hath conceived - Compare Job 15:35. The allusion here is obvious. The meaning is, when the desire which we have naturally is quickened, or made to act, the result is that sin is produced. As our desires of good lie in the mind by nature, as our propensities exist as they were created, they cannot be regarded as sin, or treated as such; but when they are indulged, when plans of gratification are formed, when they are developed in actual life, the effect is sin. In the mere desire of good, of happiness, of food, of raiment, there is no sin; it becomes sin when indulged in an improper manner, and when it leads us to seek that which is forbidden - to invade the rights of others, or in any way to violate the laws of God. The Rabbis have a metaphor which strongly expresses the general sense of this passage" - "Evil concupiscence is at the beginning like the thread of a spider's web; afterwards it is like a cart rope." Sanhedrin, fol. 99.
It bringeth forth sin - The result is sin - open, actual sin. When that which is conceived in the heart is matured, it is seen to be sin. The design of all this is to show that sin is not to be traced to God, but to man himself; and in order to this, the apostle says that there is enough in the heart of man to account for all actual sin, without supposing that it is caused by God. The solution which he gives is, that there are certain propensities in man which, when they are suffered to act themselves out, will account for all the sin in the world. In regard to those native propensities themselves, he does not say whether he regards them as sinful and blameworthy or not; and the probability is, that he did not design to enter into a formal examination, or to make a formal statement, of the nature of these propensities themselves. He looked at man as he is as a creature of God - as endowed with certain animal propensities - as seen, in fact, to have strong passions by nature; and he showed that there was enough in him to account for the existence of sin, without bringing in the agency of God, or charging it on him.
In reference to those propensities, it may be observed that there are two kinds, either of which may account for the existence of sin, but which are frequently both combined. There are, first, our natural propensities; those which we have as men, as endowed with an animal nature, as having constitutional desires to be gratified, and wants to be supplied. Such Adam had in innocence; such the Saviour had; and such are to be regarded as in no respect in themselves sinful and wrong. Yet they may, in our case, as they did in Adam, lead us to sin, because, under their strong influence, we may be led to desire that which is forbidden, or which belongs to another. But there are, secondly, the propensities and inclinations which we have as the result of the fall, and which are evil in their nature and tendency; which as a matter of course, and especially when combined with the former, lead to open transgression. It is not always easy to separate these, and in fact they are often combined in producing the actual guilt of the world. It often requires a close analysis of a man's own mind to detect these different ingredients in his conduct, and the one often gets the credit of the other. The apostle James seems to have looked at it as a simple matter of fact, with a common sense view, by saying that there were "desires" (ἐπιθυμίας epithumias) in a man's own mind which would account for all the actual sin in the world, without charging it on God. Of the truth of this, no one can entertain a doubt. - (See the supplementary note above at James 1:14.)
And sin, when it is finished bringeth forth death - The result of sin when it is fully carried out, is death - death in all forms. The idea is, that death, in whatever form it exists, is to be traced to sin, and that sin will naturally and regularly produce it. There is a strong similarity between this declaration and that of the apostle Paul Romans 6:21-23; and it is probable that James had that passage in his mind. See the sentiment illustrated in the notes at that passage, and Romans 5:12 note. Any one who indulges in a sinful thought or corrupt desire, should reflect that it may end in death - death temporal and eternal. Its natural tendency will be to produce such a death. This reflection should induce us to check an evil thought or desire at the beginning. Not for one moment should we indulge in it, for soon it may secure the mastery and be beyond our control; and the end may be seen in the grave, and the awful world of woe.

When lust hath conceived - When the evil propensity works unchecked, it bringeth forth sin - the evil act between the parties is perpetrated.
And sin, when it is finished - When this breach of the law of God and of innocence has been a sufficient time completed, it bringeth forth death - the spurious offspring is the fruit of the criminal connection, and the evidence of that death or punishment due to the transgressors.
Any person acquainted with the import of the verbs συλλαμβανειν, τικτειν , and αποκυειν, will see that this is the metaphor, and that I have not exhausted it. Συλλαμβανω signifies concipio sobolem, quae comprehenditur utero; concipio foetum; - τικτω, pario, genero, efficio; - αποκυεω ex απο et κυω, praegnans sum, in utero gero. Verbum proprium praegnantium, quae foetum maturum emittunt. Interdum etiam gignendi notionem habet. - Maius, Obser. Sacr., vol. ii., page 184. Kypke and Schleusner.
Sin is a small matter in its commencement; but by indulgence it grows great, and multiplies itself beyond all calculation. To use the rabbinical metaphor lately adduced, it is, in the commencement, like the thread of a spider's web - almost imperceptible through its extreme tenuity or fineness, and as easily broken, for it is as yet but a simple irregular imagination; afterwards it becomes like a cart rope - it has, by being indulged produced strong desire and delight; next consent; then, time, place, and opportunity serving, that which was conceived in the mind, and finished in that purpose, is consummated by act.
"The soul, which the Greek philosophers considered as the seat of the appetites and passions, is called by Philo το θηλυ, the female part of our nature; and the spirit το αρῥεν, the male part. In allusion to this notion, James represents men's lust as a harlot; which entices their understanding and will into its impure embraces, and from that conjunction conceives sin. Sin, being brought forth, immediately acts, and is nourished by frequent repetition, till at length it gains such strength that in its turn it begets death. This is the true genealogy of sin and death. Lust is the mother of sin, and sin the mother of death, and the sinner the parent of both." See Macknight.

Then when lust hath conceived, it bringeth forth n) sin: and sin, when it is finished, bringeth forth death.
(n) By sin, in this place, he means actual sin.

Then when lust hath conceived,.... A proposal of pleasure or profit being made, agreeable to lust, or the principle of corrupt nature, sinful man is pleased with it; and instead of resisting and rejecting the motion made, he admits of it, and receives it, and cherishes it in his mind; he dallies and plays with it; he dwells upon it in his thoughts, and hides it under his tongue, and in his heart, as a sweet morsel, and forsakes it not, but contrives ways and means how to bring it about; and this is lust's conceiving. The figure is used in Psalm 7:14 on which Kimchi, a Jewish commentator, has this note;
"he (the psalmist) compares the thoughts of the heart "to a conception", and when they go out in word, this is "travail", and in work or act, this is "bringing forth".''
And so it follows here,
it bringeth forth sin; into act, not only by consenting to it, but by performing it:
and sin, when it is finished: being solicited, is agreed to, and actually committed:
bringeth forth death; as the first sin of man brought death into the world, brought a spiritual death, or moral death upon man, subjected him to a corporeal death, and made him liable to an eternal one; so every sin is deserving of death, death is the just wages of it; yea, even the motions of sin work in men to bring forth fruit unto death. Something like these several gradual steps, in which sin proceeds, is observed by the Jews, and expressed in much the like language, in allegorizing the case of Lot, and his two daughters (i);
"the concupiscent soul (or "lust") stirs up the evil figment, and imagines by it, and it cleaves to every evil imagination, "until it conceives a little", and produces in the heart of man the evil thought, and cleaves to it; and as yet it is in his heart, and is not "finished" to do it, until this desire or lust stirs up the strength of the body, first to cleave to the evil figment, and then , "sin is finished"; as it is said, Genesis 19:36.''
(i) Midrash Haneelam in Zohar in Genesis. fol. 67. 4.

The guilty union is committed by the will embracing the temptress. "Lust," the harlot, then, "brings forth sin," namely, of that kind to which the temptation inclines. Then the particular sin (so the Greek implies), "when it is completed, brings forth death," with which it was all along pregnant [ALFORD]. This "death" stands in striking contrast to the "crown of life" (James 1:12) which "patience" or endurance ends in, when it has its "perfect work" (James 1:4). He who will fight Satan with Satan's own weapons, must not wonder if he finds himself overmatched. Nip sin in the bud of lust.

Then desire having conceived - By our own will joining therewith. Bringeth forth actual sin - It doth not follow that the desire itself is not sin. He that begets a man is himself a man. And sin being perfected - Grown up to maturity, which it quickly does. Bringeth forth death - Sin is born big with death.