Peacemakers who sow in peace raise a harvest of righteousness.
Does this verse fit with the rest? It almost seems like a little addendum to the main point. If we look at it alone, it makes total sense... a peacemaker, who would sow in peace, because he is a peacemaker, will reap a harvest of righteousness. Very similar to Jesus' words in the Sermon on the Mount. If we look at it in the context, it still makes sense, but in the bigger picture of heavenly wisdom. Peace-making is a sign of true wisdom that is from above.