There are two theological terms describing the concept of the bread and wine becoming the Body and Blood of Jesus Christ; one is called consubstantiation, and the other is called transubstantiation.
26 While they were eating, Jesus took bread, and when he had given thanks, he broke it and gave it to his disciples, saying, “Take and eat; this is my body.”
27 Then he took a cup, and when he had given thanks, he gave it to them, saying, “Drink from it, all of you. 28 This is my blood of the covenant, which is poured out for many for the forgiveness of sins.
Understanding the two concepts:
Consubstantiation is when two or more items coexist in union with each other. In this concept, the items co-occur yet remain separate. Think of mixing olive oil and water. They are in relationship with each other: olive oil and water; a new mixture. Blended together, both the water and oil remain independent yet co-exist together. They can be later separated, or they can exist suspended in solution together. Consubstantiation is the concept of the bread and wine co-existing with the Body and Blood of Christ. Here, the bread and wine is not the Body and Blood; they are present with the Body and Blood.
Transubstantiation is that the bread and wine is the Body and Blood of Christ. The substance of the bread and wine has now changed.
Imagine a chair. What makes the item a chair is not the materials of the item. It is a chair regardless of what it is made of. Whether one imagines a wooden chair, a metal chair, or a fabric chair, it is still a chair. The material it is made of is not important; it is a chair.
Another example of transubstantiation is a gift. Your grandparent has two identical watches, and one of those watches is handed to you with the words, “Here is a gift I have for you. Keep this and remember me when you wear it.” It is now a gift; its essence is forever different from the other watch. Of course, the “gift-ness” could be a watch or something else (its form is considered a coincidence (or accident), for it could have been something else and still be his gift). The keyword “is”; opposed to “with.” The watch “is” now the gift, not “like” a gift. It has become the gift.
Excerpts from: Michael J Heiman Religious Mysteries and Other Questions Revealed ©2018