How can we be certain about God's promises to us, when they seem to have failed concerning Saul and the Jewish Priesthood, both of which failed? Augustine points out that these promises were not to the specific people and institutions immediately concerned. Saul could not have expected to live forever as King, and the priestly line would not last. Instead, we need to understand that these promises are made to the spiritual City of God, while still having some immediate historical application.
When God takes the kingdom of Israel from Saul, for example, we should understand that He is divided Spiritual Israel from earthly Israel, just as He divides the City of God from the city of man.