After the consecration of the bread and of the wine, the bread is transmuted, transubstantiated, converted and transformed into the true Body Itself of the Lord… and the wine is converted and transubstantiated into the true Blood Itself of the Lord, Which as He hung upon the Cross, was poured out for the life of the world. (Pan-Orthodox Council of Jerusalem +1672, Decree 17)
That the Bible speaks of “transubstantiation” actually isn’t difficult to prove at all, when we simply consider what the word means. At its core, all it means for God to “transubstantiate” something is for Him to take the nature or substance of that thing, and replace it with a different nature or substance. The first time we see this happen is in Exodus 7:17 when God promises, “I will strike the water that is in the Nile, and it shall turn into blood.” When commenting on this passage, the 1st century Jewish writer Philo describes the water changing to blood as “transelementation” (μεταστοιχειοῦσι), and likewise the 5th century St. Leonitus of Jerusalem actually uses the word “transubstantiation” (μετουσίωσιν) to describe this miracle. Whichever word you prefer, the point is clearly the same: by a divine miracle, God changed the element or nature of water into the element or nature of blood. Thus, the Bible teaches transubstantiation!
Okay, maybe you’re not convinced. So what if God performed the miracle of transubstantiation in His first plague against the Egyptians, what does this have to do with the “medieval doctrine” concerning the eucharistic transformation? Well, consider the context of the ten plagues, and how the NT makes use of it. In Exodus 3:12-17, God reveals His Name, “I am who I am,” to Moses, and calls him to make this Name “known” to all the earth through a series of “signs,” i.e. the ten plagues (cf. 7:3-5, 9:16). This revelation of the Name then culminates in Exodus 34:5-7, when the Name of God is fully manifested in a vision of divine glory. This is important because, as Seraphim Hamilton documents, St. John draws on all of this imagery in his Gospel.
John’s prologue opens by telling us that Jesus has come to make the “glory” of the Father “known” to the world; and in order to present this, John structures Jesus’ ministry such that He identifies Himself with the divine Name, “I am,” seven times (John 4:26, 6:20, 8:24, 8:28, 8:58, 13:19, 18:5-7), describes Himself with seven “I am the” statements (6:35, 8:12, 10:7, 11:25, 10:11, 14:6, 15:1), and He performs seven “signs” that “manifest His glory” (2:1-11, 4:46-54, 5:1-15, 6:5-14, 6:16-24, 9:1-7, 11:1-45). This clearly shows that the seven “signs” Jesus performs to manifest His glory in John’s Gospel, correspond to the ten “signs” Moses performs to manifest God’s glory in the book of Exodus.
With this in mind, consider the first of these signs in both cases. For Moses, it was the transformation of water into blood, and for Jesus, it was the transformation of water into wine at the wedding of Cana. This tells us that Jesus’ miracle of “changing” water into wine is intentionally trying to remind us of God’s miracle of transubstantiating water into blood during the Exodus; and as I’ve shown at length before, the connection between blood and wine goes all the way back to Genesis, and it’s brought to its fullest culmination at “the supper” (13:2-4, 21:20), when Jesus brings out wine and alludes to Exodus 24:8, “this is my blood of the new covenant” (Matthew 26:28). Clearly, the Bible connects the transubstantiation of water into blood during the Exodus, the change of water into wine at Cana, and the transformation of wine into blood at the Last Supper.
If there’s any doubt about this, consider the fact that the next time John speaks of “the supper” is when it’s also a “γάμου” or “wedding” (Revelation 19:9), and this is the only time John uses this word outside of his description of the wedding at Cana. This means that whatever miracle happened at this wedding, something like it also happens at “the wedding supper of the Lamb,” i.e. the Eucharist. In other words, if transubstantiation was performed by God during the Exodus, and at the wedding of Cana, then it’s also performed by Him at the Supper. Is it any wonder, then, why Fathers such as Sts. Cyril of Jerusalem and Ambrose of Milan explicitly appeal to the miracles in Egypt and Cana when explaining the mystery of the eucharistic transformation?