Everything in the universe and the universe itself exists because God ideates them. Thomist theologian Fr. Austracio Nicanor says that the moment God stops ideating (thinking of) us, we would immediately disappear back into the nothingness from which he called us. We also read in the Catechism [CCC301] that God upholds and sustains the universe (by his thought).
What is a “being”?
In my view every coherent entity (material or spiritual) that exists constitutes a “being” [Coherent: forming a unified whole]. Even a cartoon character is a being, albeit an imaginary one. There could be beings within beings (think of a Russian nesting doll).
What is “substance”?
Substance and being are intimately related. Every “being” has a “substance”, where “substance” is the CONCEPT in the mind of the author/creator of that being.
“Accidents” refer to the physical/visible/tangible features of material beings. Beings can exist without accidents but they cannot exist without substance.
Now come to the Transubstantiation:
Consider the bread and wine beings lying on the altar just moments before the consecration. Up till then God also considers them as just bread and wine. When the priest pronounces the Words of Institution over them, God stops thinking of them as bread and wine. He removes those ideas/concepts from His mind. In keeping with the Thomist logic explained earlier, they now ought to collapse into nothingness as they have become devoid of substance. But they do not, because instantly God begins to see them as the Body of Christ and the Blood of Christ respectively. He basically switches one concept in his mind into another. By virtue of the switch, the Body of Christ (idea) and the Blood of Christ (idea) become the new substances of the beings which were hitherto bread and wine. This is what prevents them from collapsing. Thus, Transubstantiation exactly means a concept shift in the mind of God in relation to the Eucharistic species. As far as the human observer is concerned, he doesn’t detect any change in the accidents/species because their original atomic structure is preserved.
Next what happens is that the Eucharistic species is consumed by the recipient and gets acted on by the saliva in his mouth and the digestive juices in his stomach. In the process, the species lose their original form. Or take the case where the species becomes mouldy or fungus infested. God simply stops conceptualising them as the Body of Christ and the Blood of Christ and reverts to the original ideas of them, viz. as bread and wine. Instantaneously, their substances also change back accordingly. This is in keeping with Church doctrine which says that the Real Presence remains only as long as the species retain their original form.
God is sovereign. He does what He pleases. If so, what’s to prevent him from conceptualising a wafer of bread and a vial of wine as the Body of Christ and the Blood of Christ respectively? Can anybody argue with him?
When the celebrant holds up the consecrated host and says “Behold the Lamb of God…”, he’s inviting us to look at it through the eyes of the Father, who sees His Son and only His Son over there.