Old Christian
/ˌəʊl(d) ˈkrɪstʃ(ə)n/noun
In Spain, and subsequently other (chiefly southern European) countries: a person born into the Christian (especially the Catholic) faith, as opposed to a convert. Compare New Christian. Now historical.
Origin
Early 17th century; earliest use found in Thomas Shelton (fl. 1598–1629), translator. From old + Christian.