Unitarians are weird. They don't believe in the trinity, but are Christian, but also merged with a non-Christian sect to form Unitarian Univalersalism.

But also, Catholics and most Protestants have a very different view on the Eucharist. Catholics believe in Transubstantiation, where the bread and wine become the literal body and blood of Christ in the Eucharist, whereas many Protestant sects either see the Eucharist as symbolic of the body and blood and Christ's sacrifice, or something in between (Consubstantiation or Calvin's very complicated take).

It also helps explain modern Catholocism's stance that Protestants aren't allowed to take part in the Eucharist. It's not just a remnant of the old adversarial relationship between the sects, but different beliefs on what the sacrament is. They want everyone taking part in the ritual to understand it.