On views on which lying is sometimes permissible, lying to save one’s life from unjust persecution is a paradigm case of permissible lying. But Peter’s lies about his connection to Jesus—his famous three-fold denial of Jesus—fall precisely under that head. So if it is sometimes permissible to lie, it is hard to see how Peter acted wrongly.
Of course, even if lying is sometimes permissible, the purpose behind the lie can be wrong. Was that the case for Peter? I doubt it. Peter’s purpose was not to be suspected of being one of Jesus’s followers. Suppose that he chose a different means to that end, say by dressing in a non-Galilean way and affecting a non-Galilean accent. There would be nothing at all morally wrong with that—that’s presumably the sort of thing missionaries in repressive countries do all the time, without anybody (other than the repressive regime!) thinking it’s wrong.
Perhaps the difference in purpose is the one between (a) Peter not being thought to be one of Jesus’s followers and (b) Peter being thought to not be one of Jesus’s followers. Maybe if Peter affected non-Galilean dress, he would merely be intending (a), whereas his lies were done with the intention of (b). And maybe there is in general something wrong with intending to be thought not to be connected with Christ. Note first, however, that the defender of the permissibility of lying cannot say that the problem is with the intention to deceive. For paradigm cases of lies thought to be permissible are precisely ones where there is an intention to deceive (Nazi at the door cases, say). Second, apart from general worries about the permissibility of intentionally causing false belief, it does not seem plausible to think that it is always wrong to intend to be thought unconnected with Christ. Third, Peter need not have had intended (b): he might simply have intended (a) or he might have intended something in between—that the people he talked to would on balance have evidence that he is not connected to Christ. It does not seem that these subtle distinctions are in play in the Gospels, given that the texts do not tell us which thing Peter intended.
Maybe, though, one can argue that Matthew 10:33 (“If anyone denies me before human beings, I will deny him before my Father who is in heaven”) constitutes a special divine command, a sui generis prohibition on lying about one’s connection to Christ. That’s probably the best move for the defender of the permissibility of lying to make. I think there are some problems with this move.
First, we should limit the invocation of special divine commands that go over and beyond the natural law. We should do so both on the grounds of Ockham’s razor as well as on theological grounds. It seems that the crucial difference between the life of the Christian and Old Testament law is that the latter includes many divine commands that go over and beyond the natural law.
In fact, I like the hypothesis there are very few—and perhaps no—divine commands applicable to all Christians beyond the natural law. One might think that, say, the command to be baptized is such. But I am inclined to think not. There are consequences of baptism—grace and the forgiveness of sins. And there are consequences of refusal to be baptized—lack of the grace and the forgiveness of sins. The virtue of prudence requires of us to be baptized, but there need not be any separate divine command. There is, of course, the authority of the Church: we are to obey the elders. However, that is an instance of the authority a community has over its members for the common good of the community. (This community is a special supernatural one, of course.)
Second, the context of Matthew 10:33 is the persecution that the Church will endure. Thus if a new command is being promulgated, it seems likely to be directed at future times when the Church needs to be spreading the Gospel (hence the verse before, about acknowledging Christ before human beings). But Peter’s denial is not a part of that time. The Church has yet to be founded: the death and resurrection of Christ have not yet happened and the Holy Spirit has yet to be sent.
Of course, those of us who think all lying is wrong still have a puzzle. A lie in order to escape unjust persecution even if wrong seems to be a very minor wrong. But the Gospels do not present Peter’s denial as a minor wrong. So there is still the puzzle of where the gravity of Peter’s sin comes from. But here the task seems not to be so difficulty. It is reasonable to think of certain kinds of settings as greatly multiplying the gravity of an offense. To steal something worth less than a day’s wages is a venial sin according to reputable moral theologians. But to steal from a church a cheap mass-produced icon that is worth less than a day’s wages turns the theft into a sacrilege, a much more serious offense. The gravity is explained by the fact that it is a sacrilege, but the wrongness is explained by the fact that it is a theft—if the pastor gave one the icon, one could permissibly take it away and it would have been neither theft nor sacrilege. Similarly, pickpocketing in church is a more serious offense. Thus, I think we can say that Peter’s denial was wrong simply because it was a lie. But it was as wrong as it was because it was a lie about Peter’s affiliation with Jesus.