Peter was crucified upside down because he felt unworthy to die like Jesus

The Bible itself doesn't say how Peter died. John 21:18-19 has Jesus telling Peter that in old age "someone else will dress you and lead you where you do not want to go," which the Gospel writer says pointed to "the kind of death by which he would glorify God." That's widely read as pointing to crucifixion. It says nothing about the cross being inverted.

The earliest source for the upside-down detail is the Acts of Peter, an apocryphal narrative usually dated to around 180 to 190 AD, well over a century after Peter's death. Origen's commentary on Genesis, written in the 230s, reports the same detail, that Peter asked to be crucified head downward because he didn't consider himself worthy to die the way his Lord did. We only have that Origen material because Eusebius quotes it in his Church History around 324 AD. Tertullian, writing earlier still around 204 AD, confirms Peter was crucified in Rome under Nero but says nothing about the position of the cross.

So the martyrdom itself rests on decent, independent early attestation. The specific, moving detail, upside down because unworthy, traces back only to an apocryphal text and a report of a tradition, both written generations after the event with no named eyewitness anywhere in the chain. That's a real and useful distinction: "we have good reason to think Peter was martyred in Rome" is a different, stronger claim than "we know exactly how the cross was positioned." The second one is tradition, traceable and plausible, but not verified history.