John Calvin Commentary James 1:15

"Then the lust, when it hath conceived, beareth sin: and the sin, when it is fullgrown, bringeth forth death." — James 1:15 (ASV)

Then when lust has 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 shows, vicious broods are conceived, which eventually 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, it is not unreasonable if it is taken here, as in many other places, for actual sin.

And the Papists ignorantly lay hold of this passage and seek to prove from it that vicious, indeed, filthy, wicked, and the most abominable lusts are not sins, provided there is no assent. For James does not show 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 shows that the consummation of sin is eternal death, that sin arises from depraved desires, and that these depraved desires or affections have their root in lust. Hence, it 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. This refutes the folly of those 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 addresses; but his object was only this: to teach that there is in us the root of our own destruction.