What would happen to me if I preached that the Eucharist (communion)

There was a knight who did this and he got excommunicated
He was the first person in history to do so.
He later recanted his views and died in good standing with the church though

Depending on how charasmatic you are i think. If not important the Church will probably just tell you why you are wrong and maybe kill you if you continue. Probably likelier a mob killsyoufor it.