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there are those that claim to be part of the same Church that existed for the first 15 centuries after Christ, but it can easily be shown that they could not possibly be, since they would be able to worship in that Church, just as they don’t worship in that Church today. Justin Martyr explained this very well in his work defending the Faith to the Roman Emperor:

Justin Martyr, Apology, I.66-67, 2nd century:
Communion in the Body and Blood of Christ

It is allowed to no one else to participate in that food which we call Eucharist except the one who believes that the things taught by us are true, who has been cleansed in the washing unto rebirth and the forgiveness of sins and who is living according to the way Christ handed on to us. For we do not take these things as ordinary bread or ordinary drink. Just as our Savior Jesus Christ was made flesh by the word of God and took on flesh and blood for our salvation, so also were we taught that the food, for which thanksgiving has been made through the word of prayer instituted by him, and from which our blood and flesh are nourished after the change, is the flesh of that Jesus who was made flesh. Indeed, the Apostles, in the records left by them which are called gospels, handed on that it was commanded to them in this manner: Jesus, having taken bread and given thanks said, ``Do this in memory of me, this is my body.’’ Likewise, having taken the cup and given thanks, he said, ``This is my blood’’, and he gave it to them alone

so unless one believed in baptismal regeneration and the Eucharist is the Body of Christ, that Church would consider anyone rejecting these doctrines as they thought of the Gnostics, a heretic.