In saying to them, “Do this in memory of me,” Christ instituted His apostles as priests of the New Testament, ordering them, and their successors in the priesthood, to consecrate, offer, and distribute His Body and His Blood under the species of bread and wine, as He had just done Himself.
Priests exercise this power and accomplish this precept when, acting in the name of the person of Christ, they celebrate the sacrifice of the Mass.
In pronouncing the words of consecration, the Body and the Blood of Our Lord Jesus Christ, with His soul and His divinity, become truly, really, and substantially present under the species of bread and wine. Each part and the least fragment of these species contains Jesus Christ in His entirety, true man and true God.
“As the living Father hath sent me, and I live by the Father; so he that eateth me, the same also shall live by me. This is the bread that came down from Heaven. Not as your fathers did eat manna, and are dead. He that eateth this bread, shall live for ever” (Jn. 6:58-59).
“For as often as you shall eat this bread, and drink the chalice, you shall shew the death of the Lord, until He come. Therefore, whosoever shall eat this bread, or drink the chalice of the Lord unworthily, shall be guilty of the body and of the blood of the Lord” (I Col. 11:26-27).