There are no data that enable the time of the evangelisation in Bizkaia to be accurately fixed. Some have affirmed that the implantation of Christianity in Bizkaia was relatively late. Nevertheless, the relatively early existence of centres of Christian irradiation in ethnically and linguistically similar neighbouring towns and the lack of signs or testimonies of paganism, prevent us from putting the estimated date for the introduction of christianity back several centuries.
There is certain archaeological evidence to suggest that it began in the Low Roman Empire. Although some investigators (Vives, D'Ors) consider the gravestone of Sempronia, found in Morga and dated 362 to be Christian, it cannot be affirmed with certainty that it is, in fact, Christian.
The first well-known Christian archaeological site is a tomb in Argiñeta (Elorrio), dated 883. Contemporary burial tombs, judged to be so for their characteristic epigraphy, are numerous. The first documentary information does not appear until XI century, when churches all over the area are recorded. At first, the whole of Bizkaia, with the exception of its "autrigona" part (Encartaciones, excepting the valley of Gordexola), is supposed to have depended on Pamplona, from which it became detached in order to be incorporated into the Diocese of Alava on its creation in the XI Centry on a date that is not yet known.
The Diocese of Alava, absorbed into Calahorra in around 1088, depended on it until 1861 when the diocese of Vitoria was created. and which, in accordance with the Concordat of 1851, united the then called Basque Provinces. The autrigon area depended firstly on Oca and later on Valpuesta until it was absorbed by Burgos towards 1087. In 1754, some of the parishes that depended on Burgos were absorbed into the newly created Diocese of Santander. In 1861, all were passed to the jurisdiction of the new Vitorian seat. When in 1088 Alava was absorbed by Calahorra, there was certain resistance in Bizkaia that lasted until the XVI century . The resistance was supported by the monasterial regimes belonging to its churches and by the power of the tithe collectors. After the XVI century there was a splendid restoration thanks firstly to the work done by Bishop Luco and to the work done in the XVI century, and in the centuries that followed, by the religious Orders, especially the Jesuits and the Franciscans.
During the Middle Ages, the biscayan area was a passage for pilgrims travelling to Santiago de Compostela. This is the route known as "the coastal one". The so called "Camino de Santiago" had a very important influence on the Diocese, both in a religious sense and in a mercantile and commercial sense. Bilbao was to be one of the first capital cities to declare "Santiago" as its Patron Saint and to dedicate its main church to him. There are many Jacobean landmarks all over the province, from east to west. In 1862 the Diocese of Vitoria was established into which Bizkaia was integrated as well as Guipúzcoa and Álava. The Quo Commodius Bull on 2nd November 1949 created the Diocese of Bilbao. It was executed by the apostolic nuncio of His Holiness in Spain, monsignor Gaetano Cicognani. From its creation, the Diocese has been immersed in tense social, political and cultural situations inherited from the civil war of 1936-39. The special configuration of the Spanish political regime, born from that war, caused a very difficult civil and religious coexistence, with all kind of serious consequences.