Eulogius was a Christian priest of Córdoba — the capital of Muslim al-Andalus and the largest city in Western Europe — during the persecution of the Christian community by the Emir Muhammad I. From 850 onward forty-eight Christians of Córdoba were martyred for publicly confessing Christ or denouncing Muhammad in Arabic before the qadis, a movement Eulogius supported and chronicled.
His Memoriale Sanctorum (the 'Memorial of the Saints') is the principal source for the Córdoba martyrs and one of the most important Mozarabic Christian writings to survive. His Documentum Martyriale and Apologeticus Martyrum defended the martyrs' decision against critics within the Christian community who feared their public confessions provoked greater persecution.
Eulogius himself was elected archbishop of Toledo in 858 but could not take his see — Muslim authority refused to let him travel to it. The next year he sheltered a young Christian noblewoman, Leocritia, who had converted from Islam (a capital offense). Both were brought before the qadi; both refused to deny Christ. They were beheaded on the night of March 11, 859, the last named victims of the Córdoba persecution.