Isaiah 63:8

For he said - Yahweh had said. That is, he said this when he chose them as his unique people, and entered into solemn covenant with them.

Surely they are my people - The reference here is to the fact that he entered into covenant with them to be their God.

Children that will not lie - That will not prove false to me - indicating the reasonable expectation which Yahweh might have, when he chose them, that they would be faithful to him.

So he was their Saviour - Lowth renders this, ‘And he became their Saviour in all their distress;’ connecting this with the first member of the following verse, and translating that, ‘it was not an envoy, nor an angel of his presence that saved them.’ So the Septuagint renders it, ‘And he was to them for salvation εἰς σωτηρίαν eis sōtērian) from all their affliction.’ The Chaldee render it, ‘And his word was redemption (פריק pâriyq) unto them.’ But the true idea probably is, that he chose them, and in virtue of his thus choosing them he became their deliverer.