a promise keeper
In this passage, Isaiah is telling the people of Israel to look at the history of what the Lord has done. He uses the example of Abraham and Sarah, who were alone and of great age and were in the farthest state imaginable from having offspring. Even though they were old and barren, the Lord blessed Abraham and followed through in his promise to make him the father of many nations. This was meant as an encouragement to the people of Israel to remember how good God is.
This message is meant to comfort us immensely. When we look at our past, where we've come from, we get to see the work that God does in our lives. Once we make it through a trial or tough situation, we can usually look back and see how the Lord has been faithful to us.
a chance to remember
And now, whenever we are hit by trial or tragedy, or whenever we feel alone or lost, we can look back "to the rock from which you were hewn." We have the chance to remember how the Lord has delivered us and that should encourage us and push us to trust God even more so now.