when in Rome, do as the Romans do

The first attestation is Medieval Latin si fueris Rōmae, Rōmānō vīvitō mōre; si fueris alibī, vīvitō sīcut ibī (“if you should be in Rome, live in the Roman manner; if you should be elsewhere, live as they do there”), which is attributed to St Ambrose (c. 339–397). Robert Burton in his Anatomy of Melancholy (1621) uses the phrase “When they are at Rome, they doe there as they ſee done.”

• "But my friend says, ‘when in Rome, we must do as Rome does,’ Ah! this is the very principle of an ungodly world; and Christians have caught the spirit, and are acting up to it. Peter and Paul were in Rome.—If they had done as Rome did, they would not have suffered death there. If we were in Rome, and must do as Rome does, we must become idolaters, and papists, or lose Rome's favor, and suffer her displeasure. But we must separate ourselves from Rome, and from the world, and their wicked and God-robbing practices. We must not be conformed to the world. We must be Christians in deed and in truth."
• "I know of many instances where such persons have been under the necessity of buying or hiring slaves, just to preserve their reputation and keep up appearances; and even among a class of people who profess to be opposed to Slavery, have I known instances of the same kind, and have heard them apologize for their conduct by saying that “when in Rome, we must do as the Romans do.”"
• "The good intentions of the individual cannot control the evil operations of an organization which is essentially wrong. Men when in Rome are apt to do as the Romans do, however evil it may be; and the member of a Lodge is not likely to remain for a long time better than the Lodge itself."