During a Mass , through the act of transubstantiation , the bread and wine offered change , and are no longer bread and wine . They truly become the Body and Blood of Jesus Christ . The Fourth Council of the Lateran in 1215 had spoken of `` Jesus Christ , whose body and blood are truly contained in the sacrament of the altar under the forms of bread and wine ; the bread being changed ( transsubstantiatis ) by divine power into the body , and the wine into the blood . ''