When a Catholic receives Holy Communion, the Eucharist, at Mass they say "Amen."
That Amen is their agreement with EVERYTHING the Catholic Church teaches.
“If you reject...a teaching that is fundamental to Catholics," he said, "the real question is, then, Why would you want to take Communion? Because Communion itself says, 'I am part of this church and I embrace what it believes.'" ~ Bishop Arthur Serratelli