But I thought that Old Testament rules, especially ones about things having to do with sacrifice, not wearing garments made of two different kinds of material, planting two different kinds of seed in the same field, not eating pork, eating certain kinds of bread, etc. do not apply to Christians. So why should Jewish bread apply to Christians? Jesus was a Jew and followed Jewish law and Jewish traditions, but that doesn’t mean that we must do the same in using a kind of bread prescribed by Jewish Tradition. Jesus would not have eaten pork, either, but that does not mean that we cannot eat pork.
You are confusing the categories of the law as though all were equivalent and as if nothing from the Hebrew Scripture is of enduring value. This assertion is wrong.
Jesus, through His teaching as well as through the Apostles, made explicit that there are elements of the law transmitted through the first covenant that are not mutable, that there are elements of the law transmitted through the first covenant which find their fulfillment and perfection in Him, there is that which He Himself enunciates as the new Moses and, indeed, the Divine Law-giver, and there are elements of the law transmitted through the Hebrew Scriptures which do not apply in the new dispensation, as they were particular to preparing for the Christ’s coming.
“You shall love the Lord your God with all your heart, with all your soul, with all your mind and with all your strength and you shall love your neighbor as yourself. On these two commands hang all the law and the prophets.” This statement is immutable. The ten commandments are immutable – they are part of divine positive law, which endures, as well as part of natural law.
Jesus manifests, by revelation to Peter after the resurrection, that the Mosaic dietary laws do not apply in the new dispensation. This event endures in the minds of the apostles and it is part of what they teach orally, as part of apostolic tradition. It is also recorded in what we know as The Acts of the Apostles and therefore appears in Sacred Scripture.
It is not the disciple though who makes the determination of what is unchangeable. As the revelation to Peter demonstrates, that is proper to the Magisterium of the Church, which is guided by the Holy Spirit Who was promised to the apostles and, by extension, their successors.
So one must examine such questions about the law, its relevance, and its application through the mind of the Church, drawing upon the deposit of the faith, and as enunciated by the Magisterium.
That is a different matter from the nature of the sacraments of the new covenant and their relationship to Christ and how they are received and transmitted by the Church to whom they also have a relationship. This is true above all concerning the Eucharist.
The matter of the sacraments is stipulated by the Divine Founder. The matter of baptism is water. The Church cannot change that. She teaches that if another substance is used, it would be invalid in so far as it is not water.
The form for this sacrament is also prescribed by Jesus and, to the extent it is, the Church is circumscribed from altering it…although there is actually a latitude in this. Jesus said, “Go therefore and make disciples of all nations, baptising them in the name of the Father and of the Son and of the Holy Spirit.” Thus is derived the form of the sacrament.
However, the Western Church articulates this in the active voice: “, I baptise you in the name of the Father and of the Son and of the Holy Spirit.” The Eastern Churches articulate this in the passive voice: “The servant of God is baptised in the name of the Father and of the Son and of the Holy Spirit.” Both are valid since they comply with the divine commission.
So…to what I think is your point: We are told through the law and the observances of the rite of the Passover supper what Jesus used at the Last Supper. It is from the elements of the Passover supper that, at a specific moment and with given substances, he established the Eucharist. The nature, the essence and the purpose of the sacrament as He instituted it, and not the preceding law, governs what the Church must use and must do going forward in the celebration of the sacrament that He entrusted to His Church.
Nevertheless the preceding law and the observances of the rite of Passover enrich and inform aspects of our understanding of the Eucharist. Indeed, the Eucharist could not be fully understood or appreciated apart from considering it in the matrix out of which Christ brings it forth.
The Eucharist is the perfect fulfillment of what is glimpsed by way of type and foreshadowing in that first Passover meal some thirteen centuries before the coming of Christ
The praxis, properly understood, reflects the beautiful reality that directly links the Eucharist as we celebrate it today to the first celebration of the Passover supper as it was prescribed to Moses by God, in that moment in salvation history when the Lord passed over Egypt, and then led His people from slavery in Egypt, passing through the waters of the Red Sea to the Land which He had promised to Abraham, Isaac and Jacob and their descendants.
Those saving events were part of the history of the People of God of the first covenant and they endure perennially in the covenant established in the Blood of Christ precisely because the events of centuries before show forth, again through type and foreshadowing, the accomplishment of Redemption in the Person of Christ the Saviour.
The proper understanding of the theology of sign and of sacrament are thus indispensable to the very understanding of salvation history, and our experience of it and our living of it in our lives, in and through the sacraments. And, this appreciation of the theology of sign and sacrament should be precisely as it is understood, cherished and enunciated by the Church.