Adam Clarke's Commentary on the Bible

Verse 18 (Matthew 16:18)

Thou art Peter - This was the same as if he had said, I acknowledge thee for one of my disciples - for this name was given him by our Lord when he first called him to the apostleship. See John 1:42 .

Peter, πετρος , signifies a stone, or fragment of a rock; and our Lord, whose constant custom it was to rise to heavenly things through the medium of earthly, takes occasion from the name, the metaphorical meaning of which was strength and stability, to point out the solidity of the confession, and the stability of that cause which should be founded on The Christ, the Son of the Living God. See the notes at Luke 9:62 .

Upon this very rock, επι ταυτη τη πετρα - this true confession of thine - that I am The Messiah, that am come to reveal and communicate The Living God, that the dead, lost world may be saved - upon this very rock, myself, thus confessed (alluding probably to Psalm 118:22 , The Stone which the builders rejected is become the Head-Stone of the Corner: and to Isaiah 28:16 , Behold I lay a Stone in Zion for a Foundation) - will I build my Church, μου την εκκλησιαν , my assembly, or congregation, i.e. of persons who are made partakers of this precious faith. That Peter is not designed in our Lord's words must be evident to all who are not blinded by prejudice. Peter was only one of the builders in this sacred edifice, Ephesians 2:20 ; who himself tells us, (with the rest of the believers), was built on this living foundation stone: 1 Peter 2:4 , 1 Peter 2:5 , therefore Jesus Christ did not say, on thee, Peter, will I build my Church, but changes immediately the expression, and says, upon that very rock, επι ταυτη τη πετρα , to show that he neither addressed Peter, nor any other of the apostles. So, the supremacy of Peter, and the infallibility of the Church of Rome, must be sought in some other scripture, for they certainly are not to be found in this. On the meaning of the word Church, see at the conclusion of this chapter.

The gates of hell, πυλαι Αδου i. e, the machinations and powers of the invisible world. In ancient times the gates of fortified cities were used to hold councils in, and were usually places of great strength. Our Lord's expression means, that neither the plots, stratagems, nor strength of Satan and his angels, should ever so far prevail as to destroy the sacred truths in the above confession. Sometimes the gates are taken for the troops which issue out from them: we may firmly believe, that though hell should open her gates, and vomit out her devil and all his angels, to fight against Christ and his saints, ruin and discomfiture must be the consequence on their part; as the arm of the Omnipotent must prevail.

- Adam Clarke's Commentary on the Bible