Any advice for refuting the Constantine as first Pope argument put forth by some Protestants? I know it’s not true and, if anything, the bishop of Rome took on increased political responsibility as the Western Empire disintergrated and Constantine was much more concerned with unity than orthodoxy at Nicea. Still, the assertion seems so “out there” that I’m not sure how to begain.
For a start, Constantine was a layman - not a member of the clergy. Only members of the clergy could be elected Bishop of Rome.
Constantine could not have ben ordained a deacon, let alone a priest or bishop: he was not baptised until he was on his death-bed, & he had shed human blood; which was a canonical impediment to ordination.
There was a succession of Roman bishops throughout his life - he could no more be bishop of Rome, than you could be President of the USA

, because every time there was a vacancy, there was an election; & he was never elected, for the reasons given. Sylvester of Rome was elected bishop of Rome in 314 - he died after Constantine, on the last day of 337.
The positions of Emperor & Bishop could not be combined; & they never were.
Bishops of Rome had to be taken from among clerics resident in Rome - Constantine spent most of his reign in the East, not in Rome.
There was a Constantine who was bishop of Rome - from 708 to 715. He was a Syrian by birth; whereas the Emperor Constantine the Great whom we are discussing came from what is now Bosnia, and was born perhaps in 272, or perhaps in 288, & died in 337. He succeeded his father Constantius in 306 - the Empire was divided into four for the purposes of administration after 295; it was under the supreme rule of the Emperor Diocletian, assisted by three colleagues; Diocletian abdicated in 305 because of advancing age - & Constantine got rid of his enemy Maxentius in 312 & his colleague (& enemy) Licinius in 324.
So there is no basis at all for the idea - it is wholly impossible. The one fact that might lead people to believe it, is that an Roman Emperor and a Roman bishop are both named Constantine.
The sort of people who think C. was ever a pope, tend not to know much about canon law or the Roman Empire in the late third century. They are not representative of most Protestants - no Anglican cleric would be guilty of such howlers. ##