‘For he said, “Surely they are my people, children who will not deal falsely.” So he was their Saviour.'
In the past Yahweh had looked at His people, choosing them out and assuring Himself that because He had chosen them as His people they would not deal falsely, for were they not His own people (Exodus 6:7), yes, His children. Surely those whom He had chosen and with whom He had made His covenant would not deal falsely? (This is Isaiah's vivid way of bringing out the point that Yahweh treated them in this way. It is not to be interpreted literally).
That is why He had continually acted to save them. He had regularly been their Saviour, first in Egypt, then at the Reed Sea, and then regularly in the wilderness and in the land He had given them.