John Paul II presents the Eucharist as the heart of the Church’s life and the “source and summit” of Christian existence. Recalling his celebration of Mass in Jerusalem’s Upper Room, he connects the Last Supper, the Triduum, Calvary, and the Resurrection, insisting that the Mass makes Christ’s one redemptive sacrifice truly present rather than repeating it. The priest acts in persona Christi, while the faithful unite their lives and spiritual sacrifices to Christ’s obedience and love. Sacrifice, Real Presence through transubstantiation, and sacred banquet belong inseparably to the Eucharistic mystery. Holy Communion establishes life-giving union with Christ, gives the Holy Spirit, builds the Church as one body, and pledges future glory. Worthy reception requires conversion and reconciliation after grave sin; Sunday Mass, Eucharistic adoration, and visible communion in faith, sacraments, and governance are urged. John Paul II defends the necessity of ordained priests, sets limits on intercommunion, warns against reducing the Mass to a fraternal meal or introducing unauthorized innovations, and calls for careful inculturation. Mary, the “Woman of the Eucharist,” models faith, adoration, and self-offering, while the Eucharist sends believers toward justice, peace, service of the poor, and hope in Christ’s return.