In "Word Focus" Glenn McClure writes:

Verse 18 has been a source of controversy, and calls for a closer look. Here we need to ask the question, Does “this rock” refer to Peter, to Peter's confession, or to Christ? The contrast between Peter (petros) and rock (petra) sheds some light on the matter. Petros is a stone, loose and movable. It was used as a proper name and is used here in the masculine form. On the other hand, petra is feminine and therefore may not refer to Peter. Petra means a rock, immovable, like the one the wise man built his house upon (Matt. 7:24). Most Protestants, as well as some early Church fathers (Augustine, Jerome), feel that this refers to Peter's confession and not to Peter himself. This may be because this meaning had been abused by the Church of Rome, and therefore another meaning was sought. Yet from verse 23 we realize that Peter was neither the foundation nor the builder, but Christ alone, whom he had confessed (see also 1 Cor. 3:11).

Source: Building Christian Relationships: Adult Teacher's Insights, page 4.

In "God's Word for Today" David Woods writes:

When I was young, my brothers and I tried to organize a club with some friends. The problem was that we only saw our friends occasionally. During our first meeting, we talked through the rules and purpose of our club. A month later when we saw our friends again, we realized we would have to start our club from scratch. Another month later when we saw them again. . . . Sometimes the local church appears to be a sophisticated version of our boys' club. The loyalty of some members may be greater – or lesser – but the purpose is similar: to satisfy my spiritual needs and desires. Scripture paints a much different picture of the Church – a group of individuals redeemed by the blood of Christ, vitally connected to one another, committed to an eternal purpose.

Source: Building Christian Relationships: Adult Teacher's Insights, page 3.