The institution of the Eucharist was not a thing apart. It emerges from the rite of the Passover meal. The bread which He took and blessed and broke as the one presiding at the rite of Passover, in which He is fulfilling the law’s command by doing and partaking, is elevated to a new reality by Him: His flesh for the life of the world.
What is foreshadowed in the ritual meal, since the Exodus more than a thousand years before this night: the lamb of God that is offered in sacrifice. The lamb of God that is slaughtered at the temple. The lamb of God whose flesh nourishes at the moment of the Lord’s pass over. The lamb of God whose blood brings salvation and whose blood forestalls the divine wrath and whose blood preserves life but where it is absent there is death. Jesus is the Lamb of God, as the Gospel of John keeps reiterating, which the other lambs foreshadowed and pointed to – and so the elements of the Passover supper pass from being this symbolic lamb to being the true Lamb through the Eucharist.
The bread. The cups of wine. All these are types of that reality which Jesus institutes in the Eucharist and it is from these elements that Jesus, in the heart of performing and partaking of the Passover ritual, institutes the Eucharist.
The necessity of fulfilling completely the requirements of the celebration as well as the consumption of the Passover, of which the elements that were used to institute the Eucharist is an integral part, is what Thomas is addressing in Article 1 of Question 81.