Crypts developed from two closely related early Christian practices: burial in the underground catacombs that ringed cities such as Rome, where Christians interred their dead in tunnels cut through soft volcanic rock, and the custom, once churches could be built openly after the fourth century, of constructing a new church directly over the grave of a martyr, so that the building itself stood as a permanent architectural marker of a sacred burial site rather than the burial being incidental to an already-built church.
Where a martyr's original underground tomb could not simply be relocated, builders instead extended the church's foundations downward around it, effectively formalizing what had been a rough catacomb grave into a proper architectural chamber, a process visible at sites such as the crypt beneath Old Saint Peter's in Rome, built around the traditional site of the apostle Peter's burial. As church building matured through the early medieval period, the crypt became a standard element even for churches with no specific martyr's grave to commemorate, used more generally for the burial of clergy, patrons and community leaders, and larger medieval cathedral crypts sometimes grew into extensive chambers rivaling the scale of a smaller church in their own right, occasionally housing their own subsidiary altars and even hosting separate, smaller services distinct from the main liturgy celebrated above.