Since Isaiah (and later, the New Testament writers) spoke of God’s "tested" keystone (cornerstone?) or foundation stone in terms of depending upon God rather than what anyone else says or promises, it might be good to look at the importance of "foundations" and Jesus Christ as our "tested" keystone.
I once saw a vivid demonstration of the importance of a foundation. My father’s mother, my Grandma Wilson, had a wooden-frame house set upon a concrete foundation. Shortly after Grandpa died, I was staying with Grandma and she wasn’t exactly on her game so to speak. In order to back out of her curved driveway, it was necessary to curve around the corner of the house. But Grandma was distracted and hit the corner of the house with the car, knocking the whole frame of the house so that the corner of the house shifted where she hit it and the opposite corner was hanging precariously over the side of the foundation. The weight of the frame, as it leaned off the foundation, caused stress within the house so that there were cracks in the walls and ceiling. And Grandma was devastated, she didn’t know what she was going to do. She’d lost her husband and now, it looked like she’d lost her house.
Fortunately, my Uncle Joe Davis was very resourceful. He figured out that if he could jack up the corner of the house and use a carpenter’s vice to push the frame back on the foundation, the house would be all right. The stress cracks would still be there, but things would be normalized. Grandma was so relieved! Now, I’m not sure the ceiling was ever repaired so that you couldn’t see that something happened, but all in all, it was a lesson to me.
When we fall off our “tested” foundation, even a little bit, it causes stress cracks in the rest of our lives. Our selfishness, our irresponsibility, and our tunnel-vision don’t make us look cool, pretty, or mature. The relationships are broken, the bad habits are created, and the physical, emotional, and spiritual damage is done. And before it can be repaired, we have to confess our sins and placed back on the foundation. Otherwise, the kind of judgment God speaks of in verse 17 where God drops the line and the plummet and finds us off the foundation can occur.