Jamieson, Fausset and Brown on Isaiah 63:8

8. he--Jehovah "said," that is, thought, in choosing them as His covenant-people; so "said" (Psa 95:10). Not that God was ignorant that the Jews would not keep faith with Him; but God is here said, according to human modes of thought to say within Himself what He might naturally have expected, as the result of His goodness to the Jews; thus the enormity of their unnatural perversity is the more vividly set forth.

lie--prove false to Me (compare Psa 44:17).

so--in virtue of His having chosen them, He became their Saviour. So the "therefore" (Jr 31:33). His eternal choice is the ground of His actually saving men (Ep 1:3, 4).