James 4:13 says, 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: James said that someone might make plans to go into a city and live there for a year without asking if this was God's will for them, I believe is the implication. People, even Christians, may make all sorts of future plans without asking if it is God's will, but that doesn't mean that they are going to happen.
Verse fourteen adds, 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. James then reminds us that life is very short and unpredictable, comparing it to a vapor that appears for a little while and then vanishes away. Though our souls will live forever, we cannot be sure that we physically have the next second, much less the next day, week, or year, no matter what age we might be.
Verse fifteen continues, For that ye ought to say, If the Lord will, we shall live, and do this, or that. James then tells us that we should put all or plans under the will of God. Even when we pray, we should pray for God's will to be done and not for our will or desires.
Verse sixteen states, But now ye rejoice in your boastings: all such rejoicing is evil. James said people rejoiced in their boastings and that all such rejoicing is evil. If we begin to boast about how great we are, instead of giving God the glory, then such boasting or rejoicing in our accomplishments as though we and not God are responsible for them, then that is evil.
Verse seventeen adds, Therefore to him that knoweth to do good, and doeth it not, to him it is a sin. James then tells us that if we know to do good, and don't do it, that for such a person, it is sin. We, as followers of Christ, should know what is good in God's sight, and if we don't do what we know God would have us do, then we are guilty of falling into sin once more.