In creating these churches, Constantine and his architects confronted a major challenge: what should be the physical form of the church? Clearly the traditional form of the Roman temple would be inappropriate both from associations with pagan cults but also from the difference in function. Temples served as treasuries and dwellings for the cult; sacrifices occurred on outdoor altars with the temple as a backdrop. This meant that Roman temple architecture was largely an architecture of the exterior. Since Christianity was a mystery religion that demanded initiation to participate in religious practices, Christian architecture put greater emphasis on the interior. The Christian churches needed large interior spaces to house the growing congregations and to mark the clear separation of the faithful from the unfaithful. At the same time, the new Christian churches needed to be visually meaningful. The buildings needed to convey the new authority of Christianity. These factors were instrumental in the formulation during the Constantinian period of an architectural form that would become the core of Christian architecture to our own time: the Christian Basilica.