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It is polite, and possibly also advantageous, to abide by the customs of a society when one is a visitor.
Why should an English proverb single out Rome and Roman values as especially to be emulated? Couldn't we have had a 'when in Ipswich, do as the Ipswichians do' for example? As it turns out, it's all to do with the travel arrangements of a couple of early Christian saints.
St Augustine: Letters Volume I was translated from the Latin by Sister W. Parsons and published in 1951. Letter 54 to Januarius contains this original text, which date from circa 390AD:
Cum Romanum venio, ieiuno Sabbato; cum hic sum, non ieiuno: sic etiam tu, ad quam forte ecclesiam veneris, eius morem serva, si cuiquam non vis esse scandalum nec quemquam tibi.
When I go to Rome, I fast on Saturday, but here [Milan] I do not. Do you also follow the custom of whatever church you attend, if you do not want to give or receive scandal.
Januarius, who was later canonised as a martyr saint, was Bishop of Naples at the time.
The above dates the source of the proverb to at least as early as the beginnings of the Christian church. The implied flexibility on dogma and acceptance of the religious and social practices of other cultures seems to be more akin to the contemporary Buddhist teachings of the Dalai Lama than those of present day Christian authorities.
The use of the proverb in English isn't recorded until much later - well into the Middle Ages.
Robert Burton's The Anatomy of Melancholy was first published in 1621. Burton makes oblique reference to the phrase, without using it explicitly:
like Mercury, the planet, are good with good, bad with bad. When they are at Rome, they do there as they see done, puritans with puritans, papists with papists
He was slightly predated by Henry Porter, who came a little nearer to the present day version of the proverb in his play The pleasant history of the two angry women of Abington. 1599:
Nay, I hope, as I have temperance to forbear drink, so have I patience to endure drink: Ile do as company dooth; for when a man doth to Rome come, he must do as there is done.
The Interesting letters of Pope Clement XIV [a.k.a. Lorenzo Ganganelli] were published in 1777. Letter XLIV [to Prior Dom Galliard] contains the earliest version of the proverb as currently used in English that I have found in print:
The siesto, or afternoon's nap of Italy, my most dear and reverend Father, would not have alarmed you so much, if you had recollected, that when we are at Rome, we should do as the Romans do - cum Romano Romanus eris.
The proverb is so clichéd as to have been adapted to suit many other locations - this web search brings up thousands. Its familiarity, and the expectation that everyone knows the ending, has caused it also to be used in the shortened version - 'when in Rome. '. This dates back to at least the 1930s when a play of that title, written by Charles Faber, was performed in New York.