This is a text I refer to, more or less explicitly, on a regular basis (from the NRSV this time)
14 What good is it, my brothers and sisters, if you say you have faith but do not have works? Can faith save you?
15 If a brother or sister is naked and lacks daily food,
16 and one of you says to them, ‘Go in peace; keep warm and eat your fill’, and yet you do not supply their bodily needs, what is the good of that?
17 So faith by itself, if it has no works, is dead.
18 But someone will say, ‘You have faith and I have works.’ Show me your faith without works, and I by my works will show you my faith.
19 You believe that God is one; you do well. Even the demons believe — and shudder.
20 Do you want to be shown, you senseless person, that faith without works is barren?
21 Was not our ancestor Abraham justified by works when he offered his son Isaac on the altar?
22 You see that faith was active along with his works, and faith was brought to completion by the works.
23 Thus the scripture was fulfilled that says, ‘Abraham believed God, and it was reckoned to him as righteousness’, and he was called the friend of God.
24 You see that a person is justified by works and not by faith alone.
25 Likewise, was not Rahab the prostitute also justified by works when she welcomed the messengers and sent them out by another road?
26 For just as the body without the spirit is dead, so faith without works is also dead.
Why is this a favourite passage?
The short answer is that it is a key text preventing 'faith' from turning into an idol. A faith which does not bear fruit in good work is a meaningless faith - practice gives the words their sense, to use Wittgenstein's pithy aphorism. So often religious debate gets tangled up in words when ultimately it is not the words that are important. Nor, ultimately, is it a question of beliefs about matters of fact - even the demons believe! - but only of beliefs which guide our actions. A belief which has no consequence for how we live is completely irrelevant, it is simply decoration upon our mental furniture. Verse 24 is a particularly entertaining one to quote when in discussion with extreme Protestants! (It is why Luther wanted this taken out of the Bible, and called it an 'Epistle of Straw'.)