‘For you are our father.
Though Abraham does not know us,
And Israel does not acknowledge us,
You, O Yahweh, are our father,
Our redeemer from everlasting is your name.'
So he finally reminds God of what He has revealed Himself to be. God is bound, not by what He owes His people, but by what He Himself is. And what He had in pure grace revealed Himself to be as their father. When He had come to deliver His people under Moses He had claimed that Israel was His son, His firstborn (Exodus 4:22, compare Deuteronomy 32:6, which also contains the thought of redemption). Let Him remember that and act like a father towards His son.
Then he indirectly reminds Yahweh that, however tenuous might be the fact, they are the seed of Abraham. So even though because of their sinfulness and rebellion Abraham might not give them recognition as his sons, and though Israel their ancestor might not acknowledge them as his sons, God could not behave in that way. He had made promises to them through Abraham, He had given them recognition as His son, and He had revealed Himself to be the everlasting Redeemer (Isaiah 44:24; see also Isaiah 41:14; Isaiah 43:14; Isaiah 44:6; Isaiah 47:4; Psalms 78:35). He was therefore, as it were, bound in honour to behave in that way towards them.
The plea is very powerful. Isaiah recognises the dire straits in which they are. Even their forefathers would disown them because of it. But not Yahweh, for He is their Father and promised Redeemer. He has committed Himself irrevocably.
Note how easily Isaiah turns to the thought of Abraham as lying behind all that Yahweh will do for His people. Abraham is the one who first loved Yahweh (Isaiah 41:8), the one from whom (along with his grandson Israel, the fount of the children of Israel) arose the Servant.
We note that Isaiah especially was in a position to put this argument in this way, for it was because of Yahweh's relationship towards His people that Yahweh had called him to his ministry and had promised him that there would be a holy seed (Isaiah 6:9-13).