William Burkitt on James 5:12

Observe, that an oath here is not absolutely forbidden, but restrained: Above all things, my brethren.

Note, with what vehemency and earnestness the apostle speaks, Swear not, that is, swear not vainly and rashly, swear not lightly and profanely, swear not unduly, by any of the creatures, (but by the Creator only), which was a sin that the Jews were dreadfully guilty of: But let your yea be yea; and your nay, nay: accustom yourselves to a true simplicity and plainness of speech, in affirming or denying, letting oaths alone, lest ye fall into condemnation; that is, plainly, into the condemnation of hell.

Learn hence, 1. That rash and vain swearing, or profane oaths, are a high abuse of the dreadful name of God, and a mighty provocation to him: verily there is no sin that doth more weary the patience of God, because there is no sin that doth more banish the fear of God out of our hearts.

Learn, 2. That the great end of speech being to communicate the sense of our minds to each other, we ought to use such plainness and simplicity in speaking, that we may believe one another without oaths, or more solemn religious asservations.

But yet, 3. To take an oath upon a solemn occasion, when lawfully called thereunto, is a Christian and necessary duty.