Peter, who was called a rock by Christ, refers to Christ, in turn, as living stone. If Peter is a foundational rock of the earthly Church, Christ is the living stone of the eternally living Church - the very "stuff" of which the eternal, heavenly Church is made.
At each Mass, what do we take into ourselves? The Body of Christ. That body is living stone and, if you are going to build a Church you need stone. It makes sense, then, that we take the living stone inside each of us to build ourselves up as "spiritual houses", as parts of the great eternal, mystical Church of God.
He quotes the Old Testament, including these words:
"The stone that the builders rejectedIt seems appropriate coming from Peter - not just because he was named the "rock" and not just because he was the first pope. Peter often stumbled on that living stone himself; the Truth of Jesus caught him off-guard and unprepared more than once.
has become the cornerstone, and
A stone that will make people stumble,
and a rock that will make them fall."
The same stone, he reminds us, is both a cornerstone and a stumbling block. We all stumble on our faith eventually; we all make mistakes. Peter tempted Christ, sank into the water, denied Him, and fled from His side. Peter fell
Peter reminds us that we must step back and see that block we stumble over for what it is. Attached to that block are the other stones of the Church. Unmovable. Unbreakable. Eternal.
We all stumble at some point. Its important to remember that it happens to everyone, even to the first pope. It is more important, still, to remember that when you trip over the cornerstone, you're still falling into the rest of the Church.