The narthex of a church is the entrance or lobby area, located at the end of the nave at the far end from the church's main altar. At Trinity all that is not inside the altar rail is the nave. The nave is where the people sit.

Traditionally the narthex was a part of the church building, but was not considered part of the church proper. It was either an indoor area separated from the nave by a screen or rail, or an external structure such as a porch.

The purpose of the narthex was to allow those not eligible for admittance into the general congregation (particularly catechumens waiting for Holy Baptism and penitents) to hear and partake in the service.

It is believed that the narthex is the antecedent of the hospital. The sick would often be brought to the narthex of the abbey church to be tended by monks or nuns.

The narthex would often include a baptismal font as it does at Trinity parish so that infants or adults could be baptized there before entering the nave, and to remind other believers of their baptisms as they gathered to worship.

Later reforms removed the requirement to exclude people from services who were not full members of the congregation, which in some traditions obviated the narthex. Church architects continued, however, to build a room before the entrance of the nave.