Jesus’ claim in Matthew 16:18 stating that the gates of hell will not prevail against the church has long been the subject of ecclesiastical disputes, especially about the authority of the papacy, the church’s powers of absolution, and so on.
However, the common perspective among such readings of this verse center on the claim that Jesus’ promise (be it to Peter and his allegedly continuing office, or to the church as a whole) guarantees the church indefectiblity or infallibility. This verse is the locus classicus of the claim that the church is promised, by Christ a status impeccability and constancy in faithfulness.
Now, while such a reading of this verse has certainly become well enshrined in ecclesiastical claims, it reflects a very irresponsible reflection on the text itself. Truthfully it seems to me that one can only get to a theology of ecclesial indefectiblity via this text by reading a whole host of theological claims back into it that are not there.
The language of “gates of hell” is the first thing that tends to get ignored in the way this passage is deployed. Jesus’ claim that the gates of hell will not prevail against the church is a military metaphor of a rather clear kind. If the church is set in conflict with the gates of hell, the the direction of aggression–defense flows in the direction of church–hell, not the other way round. In other words, nothing in Jesus’ claim leads us to think that the church will always withstand the onslaught of the powers of darkness and remains perpetually faithful, rather the verse implies the reverse, namely that the powers of hell will not be able to stand against the incursion of the church into its territory.
Jesus’ words should not be construed as promising the church institutional infallibility, such a sense is foreign to the military metaphor of the text. Rather Jesus is promising that the church’s apocalyptic invasion of the powers of hell will not be withstood. Nothing will stand in the way of the church’s invasion of the territory claimed by the devil. This verse promises that the mission of the church will never fail, not that the church is guaranteed institutional indefecibility. Rather the promise to the church is that its own defections (cf. Matt 16:23; Luke 22:31) will not bring about the failure of the church’s mission, grounded as it is in the sole lordship of Christ (Matt 16:16).
To be sure this implies a certain sort of theology about the church’s indefectibility, but only of the sort that promises that the church will never degrade in such a way as to render the completion of its mission impossible. Thus, this passage implies the church’s indefeatability; the church clearly will never cease to exist or be malformed to such a degree that its mission fails. However this by no means implies indefectibility, let alone infallibility. Rather it promises that our defections and unfaithfulness will never be such that the mission of the church will fail. It expresses the commitment of Christ to the church, to preserve it in its missional calling, which happens all to often despite our unfaithfulness rather than through our faithfulness.