St. John Chrysostom: The priest stands at the altar...

Yes, the priest stands at the altar, the priest stands there and makes a long supplication, not in order that fire from heaven may consume the things that lie to open view, but that grace, lighting on the Sacrifice, may thereby in flame the souls of all, and show them brighter than silver purified in the fire. 

Art thou ignorant that the soul of man could never bear this fire of the Sacrifice, but that all would be utterly consumed, were not the aid of the grace of God abundant? 

For if one would but consider how great a thing it is for a mortal, and one still clothed with flesh and blood, to be enabled to be nigh to that blessed and immortal nature, he would then see how great an honor the grace of the Spirit has vouchsafed to priests.

St. John Chrysostom