16:18 Peter. The Lord is here making a play on words; in the Greek, “Peter” is petros, meaning a small stone, whereas “rock” is petra, meaning a great rock mass, solid and immovable. Even if Jesus was speaking in Aramaic, in which both meanings are expressed simply by Cephas, He was making a distinction between the two (perhaps by gestures) which Matthew picked up and (under divine inspiration) translated by the two different Greek words.
The massive rock foundation on which Christ would build His Church was Peter’s great confession of Jesus as the Creator and the Son of the living God. Peter (representing the twelve and, indeed, all who would make the same confession) would be living stones in the church built on the foundation of such confession (I Peter 2:5; Ephesians 2:19-22).
16:18 my church. This is the first, and definitive, use of the word “church” (Greek ekklesia, or “out-called assembly”) in the New Testament. This church built by Christ clearly consists of all who acquiesce volitionally and spiritually in Peter’s great confession. This in neither an invisible church, for it is composed of real people, nor a universal church, relative to the world as a whole, but always only a “little flock” (Luke 12:32). The church can never assemble together as a whole until it gathers in heaven as “the general assembly and church of the firstborn” (Hebrews 12:23). It is represented, however, as a “local assembly” in each time and place where “two or three are gathered together in my name” (Matthew 18:20), for Christ Himself, by the Holy Spirit, is there “in the midst of them.” Normally such gatherings would be formally structured as local churches (of the 115 occurrences of ekklesia, at least 85 refer specifically to local churches), with members, officers and organized programs of winning, baptizing and teaching converts.
16:18 gates of hell. “Hell” here is the Greek hades, not the ultimate lake of fire but the present pit in the heart of the earth where the souls of the lost, as well as a host of fallen angels, are confined awaiting judgment. When Christ spoke these words, the souls of believers were also there, but during His three days in the grave, Christ stormed the gates of Hades and set these redeemed captives free, taking them with Him to paradise (Ephesians 4:8-10). In like manner, He assures those in His Church that they also can deliver lost souls from imminent confinement behind the gates of Hades as they proclaim the great confession of Christ as redeeming Son of God to all who will heed the gospel.