Charles H. Spurgeon on Psalms 110:4

The LORD hath sworn, and will not repent, etc. It should be diligently considered, that God has consecrated Christ, priest by an oath, and that this was done for our sakes; First, That we might know how exceedingly momentous was this transaction, and the more reverently and with the stronger faith believe it. Secondly, That we might acknowledge the goodness of God, who, being most truthful in himself, and concerning whose faithfulness it is the greatest crime to doubt, nevertheless has been pleased to speak to us not only with a bare word, but also, after the manner of men, to confirm his decree by an oath. Rivetus.