We are to speak the truth, and we are to do it by the power of Him who is Himself the truth and who must increasingly be the object of our thoughts and our utterances. 

The second perversion of the proper use of oaths by the people of Christ's time was worse. It was evasive swearing. People who were afraid to swear by the name of the Lord because they were not telling the full truth began to swear by things. And because mere things were not thought to be as significant as the name of God, this second class of oaths was not considered to be binding. Some persons swore by their own life (1 Sam. 1:26) or their health (Ps. 15:4). Others swore by the king (1 Sam. 17:55). Still others swore, as Jesus indicates, by their head, the earth, heaven, the temple, or Jerusalem (Matt. 5:34–36; cf. 23:16, 22). All such oaths were evasive. 

When God appeared to Abraham and passed between the pieces, what did God say? He said, "Know of a surety," and He went on to outline the next five hundred years of Israel's history. Think of that phrase—“know of a surety." It means that God does not want a man to know a thing halfway. He does not want us to be doubtful about our salvation or any of His promises. And so, being able to swear by none higher, He swears by Himself. 

One of the earliest examples of the taking of an oath by God is in the remarkable story of the execution of His covenant with Abraham. When God had called Abraham out of Ur of the Chaldees into Palestine He promised that the land would be his and would belong to his seed forever. Abraham had believed God implicitly, for God is the only being in the universe who cannot lie and who has never made a mistake.

This is the fourth of Christ's six great illustrations of true morality in the Sermon on the Mount: victory over anger, purity in sexual matters, faithfulness in marriage, truth, selflessness, and love. It teaches the paramount importance of speaking truthfully.