In the previous verses, James has rebuked the oppressive rich. Now he turns to the oppressed to give encouragement. He reminds them of the returning of the Lord, and that if they will be patient and have courage, the rewarding harvest will come.
Just as Jesus’s parable of the rich man and Lazarus showed the unloving rich man receiving condemnation and Lazarus being comforted, James assures them that when the Lord returns they will receive His blessings.
He also reminds them to be good to each other and not turn on each other in their suffering. He reminds them of the generations who went before and how those who suffered had been shown honor, like Job.
They needed to remember to remain strong and faithful, despite their suffering, for “the Lord is full of tenderness and mercy.”
Verse 12 seems to kind of stand alone. James is reminding them of Jesus’s words in Matthew 5:34-37. We are to be honest in all things. We should have the reputation of always being truthful and honest so that no oath is required to convince others of our words. Honesty is our default, not deceit.