Gregory the Dialogist : In this passage it should be noted that the king twice uttered an oath above concerning the killing of Jonathan, whom nevertheless, overcome by the insistence of the people, he did not kill. What does it mean that he swore, and by no means kept the assertion of his oath? But from this matter two lessons come to us, namely that we ought to be both cautious and discerning. Cautious indeed, lest we swear; discerning, if we swear to do perverse things. For he who takes care not to swear cannot possibly commit perjury. But when one swears wrongly, it is more just that the oath be abandoned than that the crimes which are sworn be carried out. Therefore let everyone be cautious before he swears, so that either he does not swear at all, or does not swear that he will do evil things. That we should indeed be cautious, the Lord teaches, saying: "Let your speech be: Yes, yes; no, no. Do not swear by heaven, nor by the earth" (Matt. 5:37). On the other hand, the reprobate are both incautious and lacking in discernment. For they often promise that they will do evil things, and do not trouble themselves to revoke their promises, as though they would incur perjury. Hence it is that Herod swore incautiously, and fulfilled the wicked oath he had uttered in the death of the Lord's forerunner (Mark 6). We ought therefore to be cautious in our decisions, but if we neglect to be cautious, our resolutions must be abandoned, not fulfilled. For to desist from a resolution in this way is not the vice of fickleness, but the virtue of discernment. Nevertheless, swearing things that must be revoked and detested is absolutely to be avoided and greatly dreaded, because when such an oath is not fulfilled, guilt is not entirely avoided, but the lesser evil is chosen. — Commentary on 1 Kings, Book 5, Chapter 4