Charles H. Spurgeon on Psalms 95:11

The word swearing is very significant, and seems to import these two things. First, the certainty of the sentence here pronounced. Every word of God both is, and must be truth; but ratified by an oath, it is truth with an advantage. It is signed irrevocable. This fixes it like the laws of the Medes and Persians, beyond all possibility of alteration and makes God's word, like his very nature, unchangeable. Secondly, it imports the terror of the sentence. If the children of Israel could say, "Let not God speak to us, lest we die, what would they have said had God then sworn against them?" It is terrible to hear an oath from the mouth but of a poor mortal, but from the mouth of an omnipotent God, it does not only terrify, but confound. An oath from God is truth delivered in anger; truth, as I may so speak, with a vengeance. When God speaks, it is the creature's duty to hear; but when he swears, to tremble. —Robert South.