"Now listen, you who say, 'Today or tomorrow we will go to this or that city, spend a year there, carry on business and make money.' Why, you do not even know what will happen tomorrow. What is your life? You are a mist that appears for a little while and then vanishes." NIV translation
James now turns to another area where we tend to brag and boast. Once again, he leaves us with the question: Who do you think you are? He reminds us that we can't boast about tomorrow and what we will do because we are not even sure that we have a tomorrow here on the earth. God is the one that has numbered our days from the beginning and knows all that He has planned for our lives.
"Instead, you ought to say, 'If it is the Lord's will, we will live and do this or that.' As it is, you boast and brag. All such boasting is evil." NIV translation
We have all done what James is describing but it is still wrong. Assuming that we have tomorrow when we are not the ones in control is an issue of pride and is boasting. It may seem like a minor point but just adding the phrase that James gave makes a big difference and can serve to remind us that God is in control and only He knows about tomorrow.
"Anyone, then, who knows the good he ought to do and doesn't do it, sins." NIV translation
Sin is not only something that we do that is against God but it can also be things that we do not do. If the Holy Spirit tells you to do something and you do not do so, then you are in disobedience to God and that, my friends, is sin.