52 But this is no time for assembling a long list of evidence. Instead, We would rather recall the firmness of faith and complete unanimity that the Church displayed in opposing Berengarius who gave in to certain difficulties raised by human reasoning and first dared to deny the Eucharistic conversion. More than once she threatened to condemn him unless he retracted. Thus it was that Our predecessor, St. Gregory VII, commanded him to swear to the following oath: "I believe in my heart and openly profess that the bread and wine that are placed on the altar are, through the mystery of the sacred prayer and the words of the Redeemer, substantially changed into the true and proper and lifegiving flesh and blood of Jesus Christ our Lord, and that after the consecration they are the true body of Christ—which was born of the Virgin and which hung on the Cross as an offering for the salvation of the world—and the true blood of Christ—which flowed from His side—and not just as a sign and by reason of the power of the sacrament, but in the very truth and reality of their substance and in what is proper to their nature." (56)
Source: Mysterium Fidei (Vatican.va)