The foundation had been laid, but we share the word of God and once we receive the rock of revelation from the father which is that Jesus is the Christ the son of the living God, then we become a part of the church. And once we are born of God, we become a living stone. And Christ who is our foundation and our rock gathers us into his arms by the word, through his people, and starts to build his church, that church is built upon that rock. Which is the revelation that Jesus is the Christ. Christ is the cornerstone upon which each living stone should follow to make the building straight
Christ was in the prophets and the apostles, they are the foundation but only because Christ was in them, and Christ is cornerstone of all of us. But that foundation can't be laid in the hearts of people, without us sharing God's word and the gospel with one another. Thats why we are living stones, alive in God, speaking his word, and living by his will, not loving our flesh and this world, putting God first in our lives, and truly following Jesus our cornerstone, he is the way, the truth, and the life, and his way is the only way to follow. No one comes to the father except through him.