We call psychological ontology the attempt to think the being of God starting from his self-revelation in the individual life of Jesus Christ. We consider the ontological identity of Jesus Christ and the way the unity of his person is conceived crucial for understanding who this Christian God is, an understanding we take as the entry point into thinking what God is. We start from Augustine’s exegesis of the two names of God and Barth’s doctrine of election, and point out internal tensions in their respective views on divine immutability and (im)passibility, and how these connect with their concept of God and their understanding of the person of Christ. The unresolved problems in both thinkers lead us beyond their ontologies to argue that the divine-human relation that ontologically accounts for Jesus Christ’s unity is from eternity that which gives identity to the second person of the Trinity. Based on this claim we propose a reconceptualization of God’s immutability which is shown to be compatible with divine suffering and passibility.