Charles Ellicott Commentary Isaiah 63:8

"For he said, Surely, they are my people, children that will not deal falsely: so he was their Saviour." — Isaiah 63:8 (ASV)

For he said ... —The words throw us back to the starting-point of God’s covenant with His people, based, so to speak, on the assumption that they would not fail utterly in the fulfillment of their promises. (Compare Exodus 19:3-6.)