These two verses give direct and specific reasons why peace is such a great benefit toward spiritual prosperity.
"Wisdom" indicates influence of heavenly origin, that is, from God. Its effect on the mind is to make it pure and chaste, not more imaginative or intelligent. Its purpose is to make the person upright, inoffensive, and good, then peaceable, etc. It disposes a person to live at peace with others. By itself, it corroborates Jesus' statement that He is willing and able to give a peace unlike the world's, a state of being not native to man.
If a person is of a pure spirit, then peace tends to follow. First, this occurs because a pure-hearted person is at peace within himself. He is therefore not self-righteously, self-centeredly, and discontentedly seeking to impose his will and way on others to control their lives. Such a person will not induce conflict.
Second, the pure-hearted person will follow Paul's advice, which he gave in two places. Romans 14:19 says, "Therefore let us pursue the things which make for peace and the things by which one may edify another." Hebrews 12:14 adds, "Pursue peace with all men, and holiness, without which no one will see the Lord." It is very difficult for people to have conflict with others who will not fight! This does not mean that we should make peace at any cost by denying truth. We can remain faithful to truth without going to war, though it might appear costly at the moment. Jesus—and many others—did it.
James goes on to say that this approach to life's relationships produces the fruit of righteousness. This phrase could mean that what is produced as a fruit is righteousness, but it can also mean the fruit that righteousness produces. The latter is preferable. The fruit of the Spirit is the fruit the Spirit produces. The fruit of repentance is the change repentance produces in one's manner of living and attitude. Some of the fruit of righteousness are the qualities James mentions in James 3:17. Righteousness is therefore the seed from which these things grow.
But a seed needs the proper conditions to germinate, grow, and produce fruit. Regardless of how good a seed is, if the conditions are not right, this process will be hindered, and it will bear poorly. The Parable of the Sower and Seed in Matthew 13:1-9, 18-23 shows this clearly. Peace is the proper condition for the fruit of righteousness, and peacemakers are the green-thumbed gardeners. Growing a good crop demands the right conditions for good seed.
So important is peace to the Christian's spiritual prosperity that God will permit a marriage to be broken by divorce where there cannot be peace. I Corinthians 7:15 says: "But if the unbeliever departs, let him depart; a brother or a sister is not under bondage in such cases. But God has called us to peace."
Divorce is usually preceded by a fairly long period of contention. It is warfare on a small scale. Living in an environment of warfare right in the home contributes little to growing in the image of the loving God of peace. It forces one to focus on himself, and at worst, it is entirely possible God will lose the person involved in such a contentious circumstance. At the very least, growth will be slow and minimal.