Jamieson, Fausset and Brown on Isaiah 63:16

16. thou ... father--of Israel, by right not merely of creation, but also of electing adoption (Is 64:8; De 32:6; 1Ch 29:10).

though Abraham ... Israel--It had been the besetting temptation of the Jews to rest on the mere privilege of their descent from faithful Abraham and Jacob (Mt 3:9; Joh 8:39; 4:12); now at last they renounce this, to trust in God alone as their Father, notwithstanding all appearances to the contrary. Even though Abraham, our earthly father, on whom we have prided ourselves, disown us, Thou wilt not (Is 49:15; Psa 27:10). Isaac is not mentioned, because not all his posterity was admitted to the covenant, whereas all Jacob's was; Abraham is specified because he was the first father of the Jewish race.

everlasting--an argument why He should help them, namely, because of His everlasting immutability.