16:16. Simon Peter answered and said: Thou art Christ, the Son of the living God.
16:17. And Jesus answering said to him: Blessed art thou, Simon Bar-Jona: because flesh and blood hath not revealed it to thee, but my Father who is in heaven.
16:18. And I say to thee: That thou art Peter; and upon this rock I will build my church, and the gates of hell shall not prevail against it.
Thou art Peter, etc.... As St. Peter, by divine revelation, here made a solemn profession of his faith of the divinity of Christ; so in recompense of this faith and profession, our Lord here declares to him the dignity to which he is pleased to raise him: viz., that he to whom he had already given the name of Peter, signifying a rock, St. John 1. 42, should be a rock indeed, of invincible strength, for the support of the building of the church; in which building he should be, next to Christ himself, the chief foundation stone, in quality of chief pastor, ruler, and governor; and should have accordingly all fulness of ecclesiastical power, signified by the keys of the kingdom of heaven. Upon this rock, etc.... The words of Christ to Peter, spoken in the vulgar language of the Jews which our Lord made use of, were the same as if he had said in English, Thou art a Rock, and upon this rock I will build my church. So that, by the plain course of the words, Peter is here declared to be the rock, upon which the church was to be built: Christ himself being both the principal foundation and founder of the same. Where also note, that Christ, by building his house, that is, his church, upon a rock, has thereby secured it against all storms and floods, like the wise builder, St. Matt. 7. 24, 25. The gates of hell, etc.... That is, the powers of darkness, and whatever Satan can do, either by himself, or his agents. For as the church is here likened to a house, or fortress, built on a rock; so the adverse powers are likened to a contrary house or fortress, the gates of which, that is, the whole strength, and all the efforts it can make, will never be able to prevail over the city or church of Christ. By this promise we are fully assured, that neither idolatry, heresy, nor any pernicious error whatsoever shall at any time prevail over the church of Christ.
16:19. And I will give to thee the keys of the kingdom of heaven. And whatsoever thou shalt bind upon earth, it shall be bound also in heaven: and whatsoever thou shalt loose on earth, it shall be loosed also in heaven.
Loose on earth.... The loosing the bands of temporal punishments due to sins, is called an indulgence; the power of which is here granted.
16:20. Then he commanded his disciples, that they should tell no one that he was Jesus the Christ.
16:21. From that time Jesus began to shew to his disciples, that he must go to Jerusalem, and suffer many things from the ancients and scribes and chief priests, and be put to death, and the third day rise again.
16:22. And Peter taking him, began to rebuke him, saying: Lord, be it far from thee, this shall not be unto thee.
And Peter taking him.... That is, taking him aside, out of a tender love, respect and zeal for his Lord and Master’s honour, began to expostulate with him, as it were to rebuke him, saying, Lord, far be it from thee to suffer death; but the Lord said to Peter, ver. 23, Go behind me, Satan. These words may signify, Begone from me; but the holy Fathers expound them otherwise, that is, come after me, or follow me; and by these words the Lord would have Peter to follow him in his suffering, and not to oppose the divine will by contradiction; for the word satan means in Hebrew an adversary, or one that opposes.