But above all things - That is be especially careful on this point; whatever else is done, let not this be. The manner in which James speaks of the practice referred to here, shows that he regarded it as a sin of a very heinous nature; one that was by all means to be avoided by those whom he addressed. The habit of swearing by various things was a very common one among the Jews, and it was important to guard those who from among them had been converted to Christianity on that subject.
Swear not - See this command illustrated in the notes at Matthew 5:33-34. Nearly the same things are mentioned here, as objects by which they were accustomed to swear, which are referred to by the Saviour.
But let our yea be yea - Let there be a simple affirmation, unaccompanied by any oath or appeal to God or to any of his works. A man who makes that his common method of speech is the man who will be believed. See the notes at Matthew 5:37.
Lest you fall into condemnation - That is, for profaning the name of God. “The Lord will not hold him guiltless that taketh his name in vain,” Exodus 20:7.