Hadrian's Pantheon had stood on the Campus Martius for nearly five centuries by the time Boniface IV became pope, a temple to all the gods with a coffered concrete dome that no builder of his own age could have raised. Rome by 609 was a diminished city under Byzantine administration, its population a fraction of the imperial figure, its temples closed by law since the end of the fourth century and increasingly stripped for building material. Boniface asked the emperor Phocas at Constantinople for the building, received it, and had it consecrated as the church of Saint Mary and the Martyrs, reportedly bringing cartloads of bones from the catacombs to lie beneath the new altar.
The consequence was structural as much as religious. Christian use kept the roof on. Almost every other temple of the city fell into ruin or was dismantled over the following centuries, while the Pantheon, protected by continuous liturgical use, survived with its dome and bronze doors, which is why it remains the best preserved building of ancient Rome. The reuse was not an isolated whim: it belongs to a wider late antique pattern in which the church absorbed the physical fabric of the old cult rather than erasing it, a pattern visible from Syracuse to Athens.
Later Western tradition draws a second line from the same day. Because the dedication honoured Mary together with all the martyrs, medieval and modern writers frequently treat it as an origin point for the feast of All Saints, which was kept in Rome in May before Gregory III and Gregory IV moved the observance to 1 November in the eighth and ninth centuries. The connection is a reasonable reading rather than a documented decree, and the older Eastern commemoration of all martyrs after Pentecost shows the idea did not begin with Boniface.