[Immutability] means that, being perfect, God cannot and does not change. In order to change, a moral being must change in either of two ways. Either he must change for the better or he must change for the worse. God cannot get better, because that would mean that He was less than perfect earlier, in which case He would not have been God. But God cannot get worse either, because in that case He would become imperfect, which He cannot be. God is and must remain perfect in all His attributes.