The city of Krk, in addition to being the administrative centre, is also the ecclesiastical centre of the island of Krk, serving as the seat of the Diocese of Krk led by a bishop. For nearly two millennia, the Diocese has encompassed not only Krk but also areas of the neighbouring islands of Cres, Lošinj, Rab, the northern part of Pag, and smaller islands in the Cres-Lošinj archipelago such as Unije, Susak, Ilovik, and Velike and Male Srakane.

The Italian Jesuit and historian Daniele Farlati (1690–1773) stated that the Diocese of Krk is as old as Christianity itself. While this assertion cannot be substantiated with written records, it is supported by other evidence, primarily the apostolic tradition. Christianity initially spread through urban centres, establishing its first communities there. By ad 425 (following Emperor Constantine I’s ad 313 Edict of Milan, which ended religious persecution in the Roman Empire), a law mandated the establishment of bishoprics in cities, each governed by a bishop. Given that Krk already had the characteristics of a city by the advent of Christianity, it qualified to establish a bishopric in the earliest days of the faith, long before the migration of peoples. Archaeological findings confirm the existence of a bishop’s cathedral as early as the 5th century, although no documents about the Diocese’s founding have survived. The early development of Marian, apostolic, and early Christian martyr cults further attests to the Church’s antiquity in Krk. The patron saint of the Diocese and the city of Krk is St Quirinus, who died a martyr’s death in ad 305 as the bishop of Sisak.

The first written mention of the Diocese of Krk dates to 680, when Bishop Andrija of Krk attended a church council (synod) in Rome alongside Istrian bishops. At the 827 council in Mantua, both the bishops of Krk and Osor were mentioned, albeit not by name. Similarly, the presence of a bishop of Krk was recorded at the church council in Split in 928, alongside other Dalmatian bishops. Bishop Vital of Krk was named in 990, and to date, the names of 57 bishops of Krk are known.

Dr Antun Mahnić (1897–1920), the most renowned bishop of Krk, founded the Old Slavonic Academy (now the Old Slavonic Institute in Zagreb) and organised the Croatian Catholic Movement. Appointed bishop in 1897, Mahnić defended Church Slavonic liturgy and the Glagolitic script, established the Kuryk printing house, and published the newspaper Pučki Prijatelj. His remains were transferred to Krk Cathedral in 2002, and his canonisation process has begun.