In bringing up this topic of honesty, James is quoting, or at least alluding to, what Jesus said in the Sermon on the Mount. Jesus told His followers their simple "yes" was to be as strong as a solemn promise. James echoes the teaching of Jesus when He says, in James 5, verse 12, Above all, my brothers, do not swear--not by heaven or by earth or by anything else. Let your "Yes" be yes, and your "No," no, or you will be condemned.