In the Old Testament we read in Exodus (27: 20-21) that God commanded that a lamp filled with the purest oil of olives should always burn in the Tabernacle.
“You are to order the sons of Israel to bring you pure olive oil for the light, and to keep a flame burning there perpetually. Aaron and his sons are to set this flame in the Tent of Meeting, outside the veil that is before the Testimony. It must burn there before the Lord from evening to morning perpetually. This is an irrevocable ordinance for their descendants, to be kept by the sons of Israel.”
In Judaism, the sanctuary lamp is known by its Hebrew name, ner tamid (נר תמיד), which means "eternal flame or light".It is to be found in front of the ark in every Jewish synagogue. It represents the menorah (branched candlestick) of the Temple in Jerusalem as well as the continuously burning fire on the altar of burnt offerings in front of the Temple. It also symbolizes God's eternal presence, and is therefore never extinguished.
The General Instruction of the Roman Missal in the Catholic Church, states (in 316): "In accordance with traditional custom, near the tabernacle a special lamp, fuelled by oil or wax, should be kept alight to indicate and honour the presence of Christ." The sanctuary lamp is placed near the tabernacle as a sign that the Blessed Sacrament is reserved or stored there.
It is a mark of honour to remind the faithful of the presence of Christ, and is a profession of their love and affection. It signifies Christ, as this material light represents He who is the "true light that enlightens all men" (John 1:9).
It is also used by some churches of other denominations to represent the presence of God and may also be found in Eastern Orthodox Churches. Other Christian denominations burn the lamp to show that the light of Christ always burns in a sin-darkened world.
Sanctuary lamps are usually red and may be in the form of a lamp hanging near the tabernacle or, as at St Ann’s, fixed to a bracket on the wall. The red glass container holds a candle that will burn for approximately one week and is never extinguished, the new candle being lit from the used candle it replaces.