1 Samuel 15:29

the Strength of Israel This word, which occurs here only as a title of God, combines the ideas of stability, permanence, constancy: the Strength or Confidence of Israel does not change as men do.

will not lie nor repent The words closely resemble Numbers 23:19. There is a verbal contradiction between this utterance and 1 Samuel 15:11, which is usually explained by saying that in 1 Samuel 15:11 the historian uses language according to the manner of men(ἀνθρωποπαθῶς), while here the prophet speaks as befits the nature of God(θεοπρεπῶς). This is only a partial solution. It is precisely because God is unchangeable, that in His dealing with men He must seem to change His action as they change their conduct. This is one aspect of the great problem which runs through all religion, how human free-will can coexist with the Divine Sovereignty. Scripture is content to state both sides of the question, and leave conscience rather than reason to reconcile them.