The edict of the Council of Carthage regarding the placing of saintly relics under all altars had been allowed to lapse but it was now revived. 
However, with the wave of church building which was now taking place in western Europe, the shortage of suitable additional relics to furnish the corresponding altars posed a problem.
The availability of martyrs had diminished since the Empire became Christian, although in Spain there were some who died for their faith at the hands of the Saracens.
There were several ways in which a relic could arrive at a church. Most obviously, the church was erected over the site of the saint’s burial. Alternatively, it could be transferred in the ceremony known as a “translatio”.
Also cases of relic theft were not infrequent and since it was reckoned that the saint chose themselves where their bones resided, these thefts were considered sacred or “furta sacra”. By extension, saints whose bones lay buried and neglected could choose how and when they were discovered: this was known as “inventio”.
The monk chronicler Radulfus Glaber writing in the early eleventh century noted that, “the relics of many saints were revealed by various signs where they had long lain hidden.”
And indeed, in the ninth, tenth and eleventh centuries a number of “inventios” of significant Biblical characters were discovered in Western Europe and seized the popular imagination. In Burgundy there was Mary Magdalene and Lazarus. In Aquitaine, the head of John the Baptist and at Compostela, the Apostle James.
Each of these became important pilgrimage shrines in their own right.