The short answer is that Catholics believe the Eucharist is the Body and Blood of Jesus because it was taught by Jesus, Sjálfur, and recorded in the Bible.
Impendulo emfushane ukuthi amaKhatholika akholelwa ukuthi uMthendeleko uMzimba neGazi likaJesu ngoba wafundiswa nguJesu, Ngokwakhe, futhi yaqoshwa eBhayibhelini.