Most likely the Eucharist was an invention of Paul after a dream he had. There is a Jewish ritual at passover that has aspects of it. There were also mystery cults that had the idea of eating your god. Paul believed that other gods existed so it's possible he had heard of these mystery cults practice and lumped it together with Jesus.

If the New Testament were written today the publishing company would be sued for copyright violations and critics would call it derivative. Not a single idea or event or word doesn't come from early sources, nothing is unique about it. Which to be fair, writing fiction isnt easy.