As this letter is written to believers, verse 13 has nothing to do with judgement for sin, for which the penalty has already been paid on the cross of Calvary. Believers who show no mercy (or kindness) to others, will not be treated mercifully when they come before the judgement seat of Christ, when their service to Him in this life is appraised.
The phrase Mercy triumphs over judgement can have four meanings:
God would rather be merciful than exercise judgement over us (Micah 7:18).
Those showing mercy to others who offend against them can stand more confidently when submitted to judgement for their own offences against God.
Judgement is more lenient over those who have shown mercy.
Mercy always prevails over judgement.
That which follows has been misused, without due regard to other clear Scripture teaching, to support the heresy that we are saved by faith plus works, called "synergism". What we have here, however, is the importance of "works", the fruit of the Holy Spirit, to prove that we are saved. It is a matter of evidence.
Salvation comes by the grace of God (Romans 3:24), through faith by man in His Son (Romans 5:1), and justification by God (Romans 8:33) by means of the power (Romans 4:25) of the blood of Christ (Romans 5:9). This is the power that raised Christ from the dead, confirming that God is satisfied. Works are the result of salvation and evidence of the reality of our faith.
A confessed faith without works of faith is not saving faith, but is a worthless, non-existent, dead statement.
We are warned against this kind of faith in other parts of Scripture, e.g. "I declare to you the gospel which I preached to you, which also you received and in which you stand, by which also you are saved, if you hold fast that word which I preached to you - unless you believed in vain." (1 Corinthians 15:1-2), and "Examine yourselves as to whether you are in the faith. Test yourselves. Do you not know yourselves, that Jesus Christ is in you? - unless indeed you are disqualified." (2 Corinthians 13:5).
This meaning is clarified in the illustration given: within a church a man is short of clothing and food, and another tells him to get dressed and fed but doesn't supply him with what he needs. These are empty words because they don't help the needy brother and they put in doubt the sincerity of who said them.
Saving faith is alive and is evidenced by works of faith, the fruit of the Spirit (Galatians 5:22, Ephesians 5:9). Professing faith is dead, it doesn't save, consequently produces no fruit, or works. It is not faith at all, just words.
True faith and works are inseparable, and this is illustrated in an imagined debate between two men, one of whom is genuinely saved and produces works of faith and another who says he has faith but does not produce works. The key word here is show: the first challenges the second to show his faith, which is something invisible, intangible. Both have faith, but of a different kind: the first has living, saving faith, and has the works to prove it, the second only says he has faith but has no evidence of it.
A confession of belief in the existence of one God isn't saving faith - it may be a good start to acknowledge this reality, but the demons know it, and tremble because they know that they are going to come to judgement before Him and be condemned because of their rebellion and misdeeds. Acknowledgement of this sort involves no committal and does not produced a transformed life. When a person truly puts his trust in the Lord, he commits his spirit, soul and body to Him, and this results in a changed life.
Two examples of true faith are given from the Old Testament. They are two people who were opposites in many senses: a) the man Abraham, a holy man of God, patriarch of the nation of Israel, known and respected by the great of his time; b) the woman Rahab, a prostitute, a Gentile Canaanite of a nation condemned to be destroyed by the Israelites, as disreputable as anyone can be.
Abraham was justified by works when his faith and obedience to God was proved to be such as to be prepared even to sacrifice his "son of the promise" Isaac upon God's command.
But in Genesis 15:6 we read that Abraham "believed in the LORD, and He accounted it to him for righteousness." He was therefore justified by faith (Romans 4:3) which produced the works as evidence. His obedience showed that his faith was not merely an intellectual belief, but a heart commitment. By his works his faith was made perfect, and they identified him as the friend of God.
God knows man's heart and justified Abraham when Abraham put his trust in Him. Abraham was justified before other men when the proof of his faith was given by his work of obedience, something visible, clear evidence of the faith within him. Man says, "Show me the reality of your faith." The only way to do this is by works.
Rahab received the Israelite spies in her house in Jericho, concealed them from her own people, then told them how to escape without being detected (see Joshua 2). She jeopardised her life in doing so, giving clear proof of her faith in the Lord God of Israel whom she wished to serve, turning her back on her own idolatrous people. She was saved by her faith (Hebrews 11:31), evidenced by her works which justified her before Joshua and the Israelites (Joshua 6:17).
Works in God's eyes do not always coincide with men's ideas of "good works": in Abraham's case it was a willingness to sacrifice his son, horrible murder, in the case of Rahab it was treason to her king and country.
If we were to remove faith from these works, they would be evil. It is the faith originating them that makes them "good" - they are the fruit of faith.
Obviously this passage does not teach salvation by works without faith, or even by faith and works. Only faith can save, but it has to be the genuine article which is proved by obedience in doing that which is pleasing to God. These are the works.
The whole matter is summarised in the last verse. Faith is compared to the human body and works are likened with the spirit (breath) which gives life to the body. The body is made of matter and is inert, lifeless and useless without the spirit. Spurious faith produces no works and is equally inert, lifeless and useless.
It is not easy to tell when one is dead, but the absence of a sign of breath on a glass before the mouth and nose is proof of death. This is a startling picture of dead faith in our churches and church members with only a name to live (Revelation 3:2).
As far as we are concerned, taking into our own perspective the two examples of works given, would we be:
1. Willing to offer the dearest thing in our lives to God, if He so demanded, like Abraham was?
2. Willing to turn traitor to the world in order to be loyal to Christ?
13 For judgement is without mercy to the one who has shown no mercy. Mercy triumphs over judgement.
14 What does it profit, my brethren, if someone says he has faith but does not have works? Can faith save him?
15 If a brother or sister is naked and destitute of daily food,
16 and one of you says to them, "Depart in peace, be warmed and filled," but you do not give them the things which are needed for the body, what does it profit?
17 Thus also faith by itself, if it does not have works, is dead.
18 But someone will say, "You have faith, and I have works." Show me your faith without your works, and I will show you my faith by my works.
19 You believe that there is one God. You do well. Even the demons believe - and tremble!
20 But do you want to know, O foolish man, that faith without works is dead?
21 Was not Abraham our father justified by works when he offered Isaac his son on the altar?
22 Do you see that faith was working together with his works, and by works faith was made perfect?
23 And the Scripture was fulfilled which says, "Abraham believed God, and it was accounted to him for righteousness." And he was called the friend of God.
24 You see then that a man is justified by works, and not by faith only.
25 Likewise, was not Rahab the harlot also justified by works when she received the messengers and sent them out another way?
26 For as the body without the spirit is dead, so faith without works is dead also.