In constructing a stone building, a cornerstone is a main support element where two walls intersect. It was critical to select a quality stone because if the cornerstone would crack, break, or crumble, a major section of the structure would be at risk and likely fall.
As such, usage of the word cornerstone has come to reference anything that is indispensable or critical.
Key verse about Cornerstone: The stone the builders rejected has become the cornerstone; (Psalm 118:22, NIV)
A lifelong student of the Bible, Peter DeHaan, PhD, wrote the 1,000-page website ABibleADay.com to encourage people to explore the Bible. His main blog and many books urge Christians to push past the status quo and reconsider how they practice their faith in every area of their lives.