From Pope John Paul II's last Holy Thursday letter

"Mysterium fidei!" Every time he proclaims these words after consecrating the bread and wine, the priest expresses his ever-renewed amazement at the extraordinary miracle worked at his hands. It is a miracle which only the eyes of faith can perceive. The natural elements do not lose their external characteristics, since the "species" remain those of bread and wine; but their "substance", through the power of Christ's word and the action of the Holy Spirit, is changed into the substance of the body and blood of Christ. On the altar, then, Christ crucified and risen is "truly, really and substantially" present in the fullness of his humanity and divinity. What an eminently sacred reality! That is why the Church treats this mystery with such great reverence, and takes such care to ensure the observance of the liturgical norms intended to safeguard the sanctity of so great a sacrament. (Letter to Priests for Holy Thursday 2005, 6)