Oliphint goes on to write a few ideas entailed, but not necessarily obvious, in this doctrine of God's immutability:Turretin's definition is more concise [than Aquinas']: "Immutability is an incommunicable attribute of God by which is denied of him not only all change, but also the possibility of change, as much with respect to existence as to will." In other words ... immutability is understood as an essential aspect of God's character, such that not only does he not change essentially, but the very possibility of essential change is denied. This is not to affirm some outside force constraining God. Rather, it it is to affirm that God is who he is, and he cannot be otherwise. He is constrained ... by his own essential nature .
Yahweh does not and cannot lie because that would imply change, and also moral decay. In connection with his care for us, his purpose for our lives, God remains ever the same. This is the central focus in the scriptural doctrine of divine immutability: the issue is not simply a metaphysical construct, but is God's unchanging faithfulness to his own being, and, therefore, to us. At the heart of this relationship lies the notion that in both blessing and cursing. God remains the same. He remains faithful to the fulness of his being, expressed in the covenant.
God's immutability is not that of a statue, which never decays and never smiles, but the changelessness of a Father whose faithfulness discloses the fulness of his being. The changlessness of God, of which Scripture speaks, arises out of the manifoldness of his character expressed in his relationship to himself and, secondarily, his faithfulness to man.
Oliphint, K. Scott. God with Us: Divine Condescension and the Attributes of God. Wheaton, IL: Crossway, 2012. Print.. 82-85