"The stone that the builders rejected has become the cornerstone" - Psalm 118
“There is a light which no darkness can extinguish, a fire that burns in the silent heart, the Stone that the world rejected and yet is the foundation of the inner Temple.” - Mary Anne Atwood
"But this physical body, which is believed to be of so little importance by those who love to dream about the mysteries of the spirit, is the most secret and valuable thing. It is the true "stone which the builders rejected," but which must become the corner-stone of the temple. It is the "stone" which is considered worthless by those who seek for a God above the clouds and reject Him when He enters their house. This physical body is not merely an instrument for divine power, but it is also the soil from which that which is immortal in man receives its strength. A seed requires the power of the sunshine to enable it to take from the earth the elements necessary for its growth, and in the same sense the spiritual body of man, receiving its nutriment from the spirit, could not unfold and develop if it were not for the presence of the physical body of man, with its elementary and elemental forces; for the physical body is comparable to the wood from which is produced the fire which gives light; there would be no light if there were nothing to burn. "The more there is wood to burn, the greater will be the combustion, and thus it is with the Lapis Philosophorum or Balsamo perpetuo in corpore humano." — Life of Paracelsus by Franz Hartmann
Possible meanings:
what is rejected by humans can be embraced by God and become central to His plan
a metaphor for Jesus, who was rejected by the religious leaders of his time but later became the foundation of the Christian faith.
in Matthew 21, Jesus says, "Have you never read in the scriptures: 'The stone that the builders rejected has become the cornerstone; this was the Lord's doing, and it is marvelous in our eyes'".
This reference is also found in other parts of the New Testament, including Mark 12, Luke 20, Acts 4, and 1 Peter 2.
"…Alchemy, said Jung, stands in a compensatory relationship to mainstream Christianity, rather like a dream does to the conscious attitudes of the dreamer. The Stone of alchemy is in many respects the stone rejected by the builders of Christian culture, demanding recognition and reincorporation into the building itself." — Stephan A. Hoeller, C.G. Jung and the Alchemical Renewal