Peter was fleeing from Rome to Damascus when he met Jesus, who said to him “I’m going to Rome to be crucified again”. And Peter went back, and was crucified with his head down. All this according to tradition - that’s the wordings I’ve always heard regarding such stories about Jesus or the Apostles that can’t be found in the Bible.
Now I find that this story
is a part of the Acts of Peter, one of the New Testament apocryphals that wasn’t included in the Bible probably because it was not considered reliable, back in approx. 4th century when the Bible as we know it was compiled, and someone decided what was in and what was out. To my understanding, these apocryphals were found on parchments or papyruses in 19th or 20th century. Some were literally dug up, others found in libraries where nobody had read them for maybe more than a thousand years - they were in languages that people couldn’t read any more, and so it took research to decipher them.
Was the idea that Peter was crucified with his head down something that entered Christian imagination and Christian art in modern times, through the discovery of these Acts of Peter and translation of the old documents? Or was there an oral tradition through over 100 years that wasn’t formally approved by the Church, that just never died out? The latter sounds improbably. But if “according to tradition” in all cases such like this one, including about Mary’s parents, simply means “according to New Testament apocryphals that were discovered in 19th/20th century” I wonder why people don’t just say that.