The Catholic Church is a sacramental Church which means deeper realities are expressed in perceptible signs. The church building is not a neutral place; it is not simply a skin for liturgical action. It is a place where prayer happens; it is a temple where sacrifice is offered.
The building is a sacramental sign: there is found the Body of Christ. The door signifies the crossing of the threshold and entry into the Garden of Eden. It is return to Paradise.