Celtic's biggest and most intensely contested rivalry is against Glasgow neighbors Rangers FC, a pairing collectively known worldwide as the Old Firm. This historic fixture is deeply rooted in social, cultural, political, and religious divides within Glasgow, pitting Celtic—traditionally associated with Catholic and Irish heritage—against Rangers, which is traditionally tied to Protestant and British unionist identity. Matches between the two clubs are among the most fiercely watched and high-stakes derbies in global association football.