Under what circumstances can the consecration of the Eucharist be rendered invalid? Can the consecration ever be rendered invalid because the priest adds to or subtracts from the Eucharistic prayers? How about if the priest privately does not believe that the bread and wine become the body and blood of Christ? If so, how can the laity ever be sure that what they are receiving is in fact the Body and Blood of the Lord?
This really isn’t a question regarding the validity of a Mass. Technically, Masses are not valid or invalid. It is the consecration that is valid or invalid.
As long as the priest conveys what the Church conveys in consecrating the bread and the wine, the consecration is valid, though illicit (unlawful) if he changes the words. Even if the priest is having a problem believing—or in fact does not believe–what the Church believes and teaches regarding the Eucharist, so long as he intends what the Church intends while pronouncing the words, the consecration is valid.
Therefore invalid consecrations are a rarity in the Church, even though illicit consecrations abound in some places. For documentation on this, check with “Fundamentals of Catholic Dogma” by Ludwig Ott.
Fr. Vincent Serpa, O.P.