Belief in Holy Communion is a universal deal breaker for Protestants as it was for Jews when they heard Our Lord directly teaching them on the Eucharist.
John 6:53, 67 recounts, "The Jews on that account argued with one another, saying, 'How can this man give us His flesh to eat?' ... From this time, many of his disciples turned back and no longer went about with Him."
Father John Hardon affirms that rejection of the Eucharist is one thing that unites all Protestants. "Of all the definitions that anyone could come up with for Protestantism, that one is the most accurate: those self-professed Christians who reject the Real Presence of Christ in the Blessed Sacrament."
As many today sweep theological disagreements under the rug in favor of forging a false ecumenism with non-Catholics, it's worthy to note that Pope Leo XIII in his 1896 encyclical Satis Cognitum rejected this faulty approach to ecumenism: "There can be nothing more dangerous than those heretics who admit nearly the whole cycle of doctrine, and yet by one word, as with a drop of poison, infect the real and simple faith taught by our Lord and handed down by Apostolic tradition."
Watch the panel discuss the necessity of accepting Christ's teaching of the Eucharist in Friday's Download—The One True Faith Revisited: Eat My Flesh. This week's topics on The Download include training up children and the Desert Fathers, among others.