You may be interested in the previous answers mentioned in the FAQ about the nature of identity and identity theft in the past. For a quick summary of those answers: identity used to be primarily defined at community level: you were not just X, but X son of Y, spouse of Z, from the village of V, a church-going member of the parish of P, the owner of three white cows, your hair was dark, and A, B, and C were respectable people who knew you and your family and could testify in your favour. And communities did not have much love for vagrants and other strange people appearing on their doorstep. Impostors did exist and there's no shortage of such stories - a much studied and debated case is that of 16th century peasant Martin Guerre/Arnaud du Tilh -, but an unknown person showing up in a community and claiming to be someone would have to work a little bit on their backstory.