NOTES for Mat 16:18
On what is the Church founded? What qualities make Simon, son of Jonah, Peter, the rock laid in the foundation of the Church? Ardent faith? Reckless decisiveness? Leadership charisma? Perhaps... Peter has all these qualities. But it seems that something else is more important. Jesus says these words not immediately after Peter has called Him the Christ, the Son of the living God. First Jesus said that Peter had not come to this confession by his own human powers, but by revelation from the Father.
Thus Jesus puts Peter's openness to God in the first place. This is really so: Peter is strikingly open. He does not always understand everything correctly or at once, he sometimes hurries, sometimes misses the mark, sometimes shows weakness, but he is open, active, and bold. And this is what the Church of apostolic times becomes: it is constantly instructed by the Holy Spirit, it does not shut itself within four walls, fearlessly bearing witness to Christ before the whole world. The Church's invulnerability to the forces of evil, the fact that "the gates of hell will not overcome it," is explained precisely by its openness to God, by its reception of the power of God, the power of the Holy Spirit, which drives away every evil.