The apse of every church holds a portal to a parallel universe, a place separated from us by time and space but envisioned right in front of the faithful. Kneel down below the apse of a church with a mediaeval mosaic and look up and you will be enveloped in the world of the Second Coming, the Final Judgement and Heaven on Earth. You are offered a glimpse of another world as though you had pushed your head through Alice's looking glass. The portal that permits this vision is the apse, from the Greek, αψις (apsis), meaning an arch. This arch, like any other, leads from one place to another. Conceptually, it should not be seen as closed off, the mosaic depicts a living reality beyond the arch.
The world beyond the apse is described in the book of Revelations, the event that all Christianity awaits, the very heart of what it meant to be a Christian. The Apocalypse. Judgment day would come, Christ would return to reward his followers, punish their enemies, and cleanse the world. Throughout the centuries christians have felt that the apocalypse was as close and as real as the apse above them as they took communion.