“Cornerstone”: a stone placed at the head of the corner and serving as the foundation for two walls (see the note on Mt 21:42).
“Christ is called the cornerstone because He joins both walls that make up the spiritual house, namely the Gentiles and the Jews, in His embrace. He binds them into one concord, abolishing the useless sacrifices of the one and changing the demonic superstition of the other into piety” (Theophylact).
“Chosen, precious”: see the note on 1 Pet 2:4. “He who believes in Him will not be put to shame,” that is, he will not be deceived in his trust in the firmness of this stone.
It will bear every structure built upon it. Whoever believes in Christ, trusting that He will save him by faith and by works born of faith, will not be put to shame in this faith and confidence.
He will receive salvation through Him. But this is so only for one who believes in Christ. For such a believer, He is the precious stone, that is, the treasure itself, since all the believer’s salvation is safely built on Him.