Acts 2:41 tells us that 3000 people were added to their numbers in one day! There were, I'm sure, many other days similar to this one and this is only the first year after the crucifixion.
By 57 A.D. when Paul ostensibly writes to the Romans, can you visualize how many folks had told their brothers, sisters, mothers, fathers, friends, neighbors, acquaintances, children, employees, and everyone else? The "Good News" spread like wildfire during this period so it is not difficult to imagine believers in far off locations without the benefit of clergy....Apostle or otherwise.
"After these things were ended, Paul purposed in the spirit, when he had passed through Macedonia and Achaia, to go to Jerusalem, 'After I have been there, I must also see Rome'".[Acts 19:21]
"And the night following the Lord stood by him, and said, Be of good cheer, Paul, for as thou hast testified of me in Jerusalem, so must thou bear witness also in Rome.".[Acts 23:11]
"For I long to see you, that I may impart unto you some spiritual gift, to the end ye may be established." [Epistle to the Romans written in Corinth Verse 1:11]
These are all evidence that as late as 57 AD, there has been no Apostle in Rome. If an apostle had been there, they would already have a spiritual gift, they would already be established, and God would not need to send Paul there to bear witness of Him.
We continue to examine the writings of Jewish historian Josephus, Roman historian Tacitus, Clement of Rome, and Justin Martyr. From them we hear no mention of Peter in Rome either. Surely someone saw something and wrote it down, or maybe they did and there was nothing to write down.
Ignatius continues to insist that Peter and Paul were there in Rome preaching once, perhaps between Paul's two imprisonments after which Peter returned to Asia Minor and on to Parthian Babylon where he wrote his two epistles and died. That is entirely possible.
But there is nothing thus far about a 25 year Petrine Bishopric in Rome or an upside down crucifixion under Nero. Thus far it is nothing but a tall Vatican tale by the masters of rhetoric.
And then there is Irenaeus who continues to insist that Linus was the first Bishop of Rome, not Peter. Even the threats of purgatory will not dissuade his testimony. The magisterium have their work cut out for them, and a lot of explaining to do.