Fire is one of the most expressive and most ancient of liturgical symbols. All the creeds of antiquity accorded a prominent place to this element whose mysterious nature and irresistible power frequently caused it to be adored as a god. The sun, as the principle of heat and light for the earth, was regarded as an igneous mass and had its share in this worship. Christianity adapted this usual belief, but denied the divine title to heat and light, and made them the symbols of the divinity, which enlightens and warms humanity. The symbolism led quite naturally to the liturgical rite by which the Church on the Eve of Easter celebrates the mystery of the Death and Resurrection of Christ, of which the extinguished and rekindled fire furnishes the expressive image. The beginning of the office also reflects ancient beliefs. The new fire is struck from a flint and is blessed with this prayer:Lord God, Almighty Father, inextinguishable light, Who hast created all light, bless this light sanctified and blessed by Thee, Who hast enlightened the whole world; make us enlightened by that light and inflamed with the fire of Thy brightness; and as Thou didst enlighten Moses when he went out of Egypt, so illuminate our hearts and senses that we may attain life and light everlasting through Christ our Lord. Amen.
The Catholic Encyclopedia