In a Nutshell
Read in context, this is a piece of advice that reminds readers who they are.
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Read 5:12 in relation to the discussion of 3:1-12 of the tongue.
Most importantly, brothers and sisters, do not swear, neither by heaven nor by earth nor by any other oath. Simply let your "Yes" be "Yes" and your "No" be "No", so that you may not put a foot wrong to fall under judgment.
To read the Epistle of James successfully, to read it with profit, to grasp the wisdom the writer conveys, requires the reader to confront the appeal of the writer to his or her context. It is the same context for us readers. As we consider this letter, and as we read this simple recommendation, we realise that he has been writing his letter with this same principle in mind. This is a letter with no frills. He says what he has to say, and leaves it with us for God to do His work in our lives. That will have to suffice. But what is this saying to us? What is the context?
Earlier on, we confronted James reminding his readers that the time of trial was no justification for attempting to redefine one's standing under heaven. Trials come, but they are to be endured with patience. That is what a disciple of Jesus has learned to expect. It is certainly not the time to presume to redefine the ways and means of the Almighty. The end to presumption makes such endurance possible.
As the letter goes on, we have read again and again how the maintenance of a patient outlook avoids neglecting God's bounty "from above". Presumptuous self-assertion is overcome. The disciple is freed from competitive rivalry, to live in the realm of God's mercy and grace. In that context, in that setting, their lives indeed unfold before the judge of all the earth - the One Abraham confessed will surely do right (Genesis 18:25). That is the context in which this admonition is set forth.
The context is clearly presented. James suggests that we indeed do what Job did and now put our hand to our mouth (see v. 12 and Job 40:4). This is the wisdom that avoids such a remonstrance that presumes too much of our feeble abilities, our precarious mortality. Can we prove ourselves right in whatever judgment we have come to? Can we even keep ourselves true to the oaths that we have so solemnly declared? By adding an oath to what we say we certainly do not add to the validity of our answer. If anything it may simply gets too close to presuming that the judge is subject to our judgment.
Here then is advice in the context where Christ Jesus has already announced vindication - complete restoration, the upholding of our life. In that context any grasping onto this or that cause recedes because God Himself grasps us by giving us new hearts, revealing to us that He simply wants us to be free to enjoy the precious fruit that will come, no doubt about it, in His time (5:7). A "Yes" or a "No" in that context says it all. Why? Because whatever has been said has been uttered in the presence of the Lord who overflows in pity for our weaknesses and is abundantly compassionate in His mercy. Go easy on yourself, says James, just say what you have to say and leave it there. God picks up the pieces.