But above all things, my brethren, swear not, neither by heaven, neither by the earth, neither by any other oath: but let your yea be yea; and your nay, nay; lest ye fall into condemnation.
But above all - as swearing is utterly alien to Christian 'endurance.'
Swear not - through impatience, to which trials may tempt you (James 5:10-11). In contrast stands the proper use of the tongue (James 5:13). Referring to Matthew 5:34, etc.
Let your yea be yea - do not use oaths in everyday conversation: let a simple affirmative or denial be enough to establish your word.
Condemnation - judgment of "the Judge" who "standeth before the door" (James 5:9).