Constructing a corner is complicated business: joining two right angle pieces together in such a way that the resulting union is stronger than either one alone. The corner piece, whether it is a pole with bracing in the case of field fencing, or a large stone in the case of a building, must be strong enough to hold and support the joined structure together.
In the case of the psalmist, he describes one such large stone that seemed such a “misfit” that it was rejected by the builders as being unusable. However, once the construction started, it was clear the stone was cut and created exactly for the purpose of becoming the foundational support, and was desperately needed for the project to be able to continue. It, in fact, fit the need perfectly.
A “cornerstone” installation has become a common ceremonial task with any new building construction to this day. Even if it has no functional significance to the strength of the building, (it is most often just a plaque with the name and date of the building), it does help bear the emotional weight of the effort made to plan, design, and fund new construction that is finally becoming reality.
It is tempting as well to treat Jesus as a ceremonial headstone of the corner, as there is plenty of emotion in the nativity story during advent. Who cannot hear the Christmas story without feeling sentimental and awestruck? But he is no ritual plaque in the side of the wall, simply a reason for a celebratory ceremony and then everyone goes home while someone else finishes the construction. Jesus is the real foundation, the only foundation we have to build on, and his birth demands that we build his church upon him, rolling up our sleeves and getting dirty and sweaty in the process. Like the headstone of the psalm, he absolutely was rejected by the pharisees as a misfit for his time, unacknowledged for the strength he bore alone in joining disparate Jews and Gentiles together who had previously been at right angles to one another.
We cannot make the same mistake again.