Francis I. Andersen and David Noel Freedman on 1 Samuel 15:29//Numbers 23:19

[We must deal] with the general scope and implication of the statement that “the Eternal One of Israel does not lie and does not repent, because he is not a man to repent” (1 Sam 15:29). The statement here is very similar to the one in Num 23:19, where it is said, “El is not a man (‘ȋš) that he should dissemble (wikazzeb) or a human being (ben-‘ādām) that he should repent (wěyitneḥām).” The accompanying line (v 19b) shows that what is meant by the statement concerns God’s reliability and consistency, “Shall he say something and not do it/or speak and not establish it?” The conclusion would be in both places that God is different from man in that he is faithful and just; he does what he says he will do. He does not say one thing and do another, neither does he change his mind for frivolous reasons or no reason. He is not capricious or arbitrary but is truthful, consistent, and reliable. In that sense he does not repent: he does not change his mind and then change it again without cause. (Francis I. Andersen and David Noel Freedman, Amos: A New Translation with Introduction and Commentary [AB 24A; New York: Doubleday, 1989], 653)