Just in case you didn’t understand what Saint James said in the previous 4 chapters, today, in the last line of Chapter 4, he makes it as clear as he can:
17 Whoever knows what is right to do and fails to do it, for him it is sin.
So failing to DO what is RIGHT (or Good) is a SIN.
What does this show us?
First, works do not spring forth from faith as evidence of faith. If faith just automatically produced works, James would not have to tell his readers that failure to do them is a sin.
Second, we have free will. James is writing to converts, thus they are Christians. But clearly they can still choose to not do what is right. If the failure to do the right thing is a sin, it follows that intentionally doing the wrong thing is also a sin. Thus, you can commit sin after your conversion. Again, these are converts and James is warning them against falling into behavior that will be sinful.
Here we should stop and ask, “what is sin?” The short answer is that it is an offense against God’s will. It is going against God’s will.
That raised the question, if you go against God’s will are you saved?