Is the tradition that St. Peter was crucified necessary doctrine? I ask specifically in regard to it being done upside-down. What is the earliest historic testimony to this aspect of his death? The reason that I ask is that it seems out of character for Romans to acquiesce to a request of this sort from a prisoner. Also, with some scholarly theories about how crosses were constructed, upside-down crucifixion would have been impossible logistically. Thanks in advance for any insights.
The detail about St Peter being crucified upside down to avoid dying in the same manner as His master is a pious tradition. I haven’t looked into the historical basis, but it is type of thing that could easily be spurious, and it is not in any sense essential to the faith.
We do have to accept that St Peter was martyred, because it is mentioned in scripture, as a prophecy, although it is quite likely that the words were penned after the event. We don’t have to accept that Jesus genuinely did predict St Peter’s death, simply because Scripture says so; no competent modern Catholic scholar is that kind of literalist, although a overly sceptical attitude is just as distorting as blind belief in everything. However a lot of Catholic theology begins to fall apart if St Peter was not martyred. However it is not a real problem - the historical evidence is as good as anyone has any right to expect that St Peter died in Rome for his faith.