This and the next paragraph denounce the vices of the rich, in the spirit of Amos and Isaiah; that they are Jews, and not Christians, seems obvious, if this epistle is to be got into the first centuries of Christian history, when the rich had small power to oppress the poor. First comes a warning suggested presumably by the Lord's parable of the Rich Fool. They make plans for a year, and know not what will happen the very next day; human life is transitory as a puff of steam. They were proud of big plans which fate might turn to folly. And such idle words (Matthew 12:36) were not meaningless frivolities; there was evil in them it is a strong word, that which closes the Lord's Prayer. Finally, since these people knew how to do good did they not boast of their Law? and would not do it, they were guilty of sin. For the NT with one consent here following the spirit of the prophets makes sin mainly the failure to do right, and not merely the doing of wrong.
Related Commentaries of James 4:13
James 4:13-17
13 Go to now, ye that say, To day or to morrow we will go into such a city, and continue there a year, and buy and sell, and get gain:
14 Whereas ye know not what shall be on the morrow. For what is your life? It is even a vapour, that appeareth for a little time, and then vanisheth away.
15 For that ye ought to say, If the Lord will, we shall live, and do this, or that.
16 But now ye rejoice in your boastings: all such rejoicing is evil.
17 Therefore to him that knoweth to do good, and doeth it not, to him it is sin.