There stands the priest, not bringing down fire from Heaven, but the Holy Spirit: and he makes prolonged supplication, not that some flame sent down from on high may consume the offerings, but that grace descending on the sacrifice may thereby enlighten the souls of all, and render them more refulgent than silver purified by fire. Who can despise this most awful mystery, unless he is stark mad and senseless? Or do you not know that no human soul could have endured that fire in the sacrifice, but all would have been utterly consumed, had not the assistance of God’s grace been great. (On the Priesthood, 3.4)
(Thanks to James for the Quote).