Where has Christ promised that he will as certainly feed and nourish believers with his body and blood as they eat of this broken bread and drink of this cup?

In the institution of the supper, which reads thus: The Lord Jesus in the night in which he was betrayed took bread; and when he had given thanks, he brake it, and said, This is my body, which is for you; this do in remembrance of me. In like manner also the cup, after supper, saying, This cup is the new covenant in my blood; this do, as often as ye drink it, in remembrance of me. For as often as ye eat this bread, and drink the cup, ye proclaim the Lord’s death till he come, 1 Cor. 11:23–26. This promise is repeated by St. Paul, where he says: The cup of blessing which we bless, is it not a communion of the blood of Christ? The bread which we break, is it not a communion of the body of Christ? seeing that we, who are many, are one bread, one body; for we all partake of the one bread, 1 Cor. 10:16, 17.

Why, then, does Christ call the bread His body, and the cup His blood or the new covenant in his blood, and Paul, a communion of the body and blood of Christ?

Christ speaks thus not without great cause; namely, not only to teach us thereby that, as bread and wine sustain this temporal life, so also his crucified body and shed blood are the true food and drink of our souls unto eternal life; but much more, by these visible signs and pledges to assure us that we are as really partakers of his true body and blood, through the working of the Holy Spirit, as we receive by the mouth of the body these holy tokens in remembrance of him; and that all his sufferings and obedience are as certainly ours as if we ourselves had in our own persons suffered and made satisfaction to God for our sins.

Article 8: For this was the sovereign counsel, and most gracious will and purpose of God the Father, that the quickening and saving efficacy of the most precious death of his Son should extend to all the elect, for bestowing upon them alone the gift of justifying faith, thereby to bring them infallibly to salvation: that is, it was the will of God, that Christ by the blood of the cross, whereby he confirmed the new covenant, should effectually redeem out of every people, tribe, nation, and language, all those, and those only, who were from eternity chosen to salvation, and given to him by the Father; that he should confer upon them faith, which together with all the other saving gifts of the Holy Spirit, he purchased for them by his death; should purge them from all sin, both original and actual, whether committed before or after believing; and having faithfully preserved them even to the end, should at last bring them free from every spot and blemish to the enjoyment of glory in his own presence forever.