An arch depends on a keystone.  It is that special stone against which both sides of the arch push; it holds the whole arch in place.  It is the stone on which the weight of the building above rests; in that regard, it is foundational.  Jesus is the Keystone.  Rest the whole structure on Him and it will stand.  “The stone that the builders rejected has become the very head of the corner” (Acts 4:11).  Make the keystone of the church to be program, personality or politics and it will fall.  The temple centered in Christ and His indwelling Spirit will survive and thrive.