This transcendent God (the God who is above and beyond creation) had made a faithful promise to the ancestors of Moses. He had kept this promise for hundreds of years.
Moses stood in awe of the one holy and transcendent God of Abraham. This is the God who set himself apart from all other gods.
Later, Jesus quoted this verse when he spoke with the Sadducees about the resurrection (Matthew 22:32; Mark 12:26; Luke 20:37).