What good is it, my brothers, if someone says he has faith but does not have works? Can that faith save him? If a brother or sister is poorly clothed and lacking in daily food, and one of you says to them, “Go in peace, be warmed and filled,” without giving them the things needed for the body, what good is that? So also faith by itself, if it does not have works, is dead. But someone will say, “You have faith and I have works.” Show me your faith apart from your works, and I will show you my faith by my works. You believe that God is one; you do well. Even the demons believe—and shudder! Do you want to be shown, you foolish person, that faith apart from works is useless? (James 2:14–20, ESV)
The message could not be more vividly clear: a genuine Christian with produce genuine fruit. Put another way: A true Christian will act like it! James confronts the myth that you can simply hold a doctrinal belief, that is, possess your beliefs like they're simply information, and yet have no evidence in your life that faith has taken hold. If I watch people who have real, physical needs and am not moved by it than I have not been changed by the Jesus who was on a daily basis moved with compassion for the sick, hurting and those in need of redemption.
James further makes a mockery of the notion that our good works and faith can be separated by reminding us that demons have some solid doctrinal beliefs. How terrifying! Reciting doctrine is not the same thing has having genuine faith. To be sure, your doctrine (i.e. beliefs) must line up with what the Bible teaches, but being able to recite weighty theological concepts is not enough. The demons have lots of knowledge about God and it even causes them to shutter, but what they don't have is faith. Our faith, if it is separated from a life of good works is useless (v.20).
For the Christian, this is immensely practical and challenging. The New Birth we experience when God saved us gives us new desires. We should want to serve him and live lives that are holy. For those who have wandered from their relationship with God and have not that desire to serve him, these verses are a sobering reminder to be certain they have a living faith (2 Cor. 13:5).
On the other hand, for the believer who has been gripped by God's grace and wants desperately to please God, it is a comfort to know that the salvation God has worked in us will inevitably produce good works (Phil 1:6, 2:13). This is certainly not to say that we sit back and do nothing -- that God has us on autopilot when it comes to righteous living. Holy living is hard work, but as Kevin DeYoung says, it is a "faith-fueled effort" (Col 1:29).