One reason for believing this is that _this was the way in which similar words had been used in the Jewish Passover, which Jesus and his disciples were then keeping_.
In the Passover service, when the head of the family distributed the bread, he always said--"This is the bread of affliction." When he distributed the flesh of the lamb, roasted for the occasion, he used to say--"This is the body of the Passover."
But every one knows, and every one admits, that the Jewish Passover was a _memorial_ service. It was kept in memory of the wonderful deliverance of their forefathers from the bitter bondage of Egypt. And the words used at that service were memorial words. And so, when Jesus, a little while before, had given to his disciples the Passover bread, saying--"This is the bread of affliction:" he did not mean to say that _that_ was the very same bread which their forefathers had eaten, in the time of their affliction in Egypt. What he meant to say was--this is the bread which you are to eat in _memory_ of your forefathers' trial and deliverance. And when he gave to each of them a piece of the sacrificial lamb, saying, "This is the body of the Passover;" he did not mean that in any mysterious, or supernatural sense, _that_ was the very lamb of which their forefathers had eaten on the solemn night of the Passover; he only meant that it was the body of which they were to eat in memory of the Passover. The Passover was a memorial service; and the words used at the Passover were memorial words.
And so, when Jesus went on, from the last Passover of the Jewish church, to the first sacramental feast of the Christian church, and began by saying, "This do in _remembrance_ of me," what else could the apostles possibly have thought, but that he intended this new service of the Christian church to be a memorial service, just as the old festival of the Jewish church had been? When he gave them the broken bread, and said, "This is my body;" they could only have understood him as meaning this is the memorial of my body. And when he gave them the cup into which he had just poured the wine, and said: "This is my blood;" they could only understand him as meaning this is the memorial of my blood. And so, the sense in which he had just before used the words employed in the Jewish festival must have led the disciples to understand them in the same way when he used similar words in the Christian sacrament. This is a good, strong reason for thinking of this sacramental feast as a memorial service.