Peter was told by Jesus that he was going to build his church. Needless to say, we know that Jesus was not referring to a religious building. The word translated church refers to any kind of official gathering that occurred when people were summoned to meet together. Jesus here indicates that he is going to summon people to form a community that belongs to him.
Who is going to call people to join this group? Jesus is, but in the main he is going to do it through his servants and through his people as they witness to who he is. Peter discovered one aspect of his role in the church, which was to evangelise.
When is Jesus going to build his church? The answer to this question is that he will do so throughout the subsequent centuries as sinners are brought into his kingdom, and he is still doing so today. This means that Peter's role in this growth was limited to the first few decades, and then he contributed no more. But Jesus keeps on adding sinners to his church.
How will Jesus build his church? He will do it as people discover who he is and what he has done for sinners. Simple statements like the one Peter had just made, even although it contained profound truth, would be one way in which Jesus build his church.
Would Jesus be opposed as he builds his church? He would, by those he calls ‘the gates of hell’. There is more than one suggestion as to what is meant here. One view says that the gates of a location were usually the places where the city rulers met to discuss matters connected to the town. In this view, Jesus says that the powers of darkness will engage in efforts to stop the growth of his kingdom. Another view is that the gates of hell refers to the power of death, and death is an opponent who always seems to win. Yet Jesus, under this meaning, is promising resurrection for those who belong to his church.
If I have to choose between the two options, I think it is probably the first because Jesus’ use of the word ‘prevail’ suggests an attack made by enemies of the church. Moreover, the word ‘prevail’ suggests a close conflict, almost like a wrestling match. But Jesus will ensure that his church will be built. The opposition will not succeed in preventing one of Christ’s people from being delivered from the state of sin.