Objection 1. It seems that this change is not wrought instantaneously, but successively. For in this change there is first the substance of bread, and afterwards the substance of Christ's body. Neither, then, is in the same instant, but in two instants. But there is a mid-time between every two instants. Therefore this change must take place according to the succession of time, which is between the last instant in which the bread is there, and the first instant in which the body of Christ is present.
Objection 2. Further, in every change something is "in becoming" and something is "in being." But these two things do not exist at the one time for, what is "in becoming," is not yet, whereas what is "in being," already is. Consequently, there is a before and an after in such change: and so necessarily the change cannot be instantaneous, but successive.
Objection 3. Further, Ambrose says (De Sacram. iv) that this sacrament "is made by the words of Christ." But Christ's words are pronounced successively. Therefore the change takes place successively.
On the contrary, This change is effected by a power which is infinite, to which it belongs to operate in an instant.
I answer that, A change may be instantaneous from a threefold reason. First on the part of the form, which is the terminus of the change. For, if it be a form that receives more and less, it is acquired by its subject successively, such as health; and therefore because a substantial form does not receive more and less, it follows that its introduction into matter is instantaneous.
Secondly on the part of the subject, which sometimes is prepared successively for receiving the form; thus water is heated successively. When, however, the subject itself is in the ultimate disposition for receiving the form, it receives it suddenly, as a transparent body is illuminated suddenly. Thirdly on the part of the agent, which possesses infinite power: wherefore it can instantly dispose the matter for the form. Thus it is written (Mark 7:34) that when Christ had said, "'Ephpheta,' which is 'Be thou opened,' immediately his ears were opened, and the string of his tongue was loosed."
For these three reasons this conversion is instantaneous. First, because the substance of Christ's body which is the term of this conversion, does not receive more or less. Secondly, because in this conversion there is no subject to be disposed successively. Thirdly, because it is effected by God's infinite power.
Reply to Objection 1. Some [Cf. Albert the Great, Sent. iv, D, 11; St. Bonaventure, Sent., iv, D, 11] do not grant simply that there is a mid-time between every two instants. For they say that this is true of two instants referring to the same movement, but not if they refer to different things. Hence between the instant that marks the close of rest, and another which marks the beginning of movement, there is no mid-time. But in this they are mistaken, because the unity of time and of instant, or even their plurality, is not taken according to movements of any sort, but according to the first movement of the heavens, which is the measure of all movement and rest.
Accordingly others grant this of the time which measures movement depending on the movement of the heavens. But there are some movements which are not dependent on the movement of the heavens, nor measured by it, as was said in I, 53, 3 concerning the movements of the angels. Hence between two instants responding to those movements there is no mid-time. But this is not to the point, because although the change in question has no relation of itself to the movement of the heavens, still it follows the pronouncing of the words, which (pronouncing) must necessarily be measured by the movement of the heavens. And therefore there must of necessity be a mid-time between every two signate instants in connection with that change.
Some say therefore that the instant in which the bread was last, and the instant in which the body of Christ is first, are indeed two in comparison with the things measured, but are one comparatively to the time measuring; as when two lines touch, there are two points on the part of the two lines, but one point on the part of the place containing them. But here there is no likeness, because instant and time is not the intrinsic measure of particular movements, as a line and point are of a body, but only the extrinsic measure, as place is to bodies.
Hence others say that it is the same instant in fact, but another according to reason. But according to this it would follow that things really opposite would exist together; for diversity of reason does not change a thing objectively.
And therefore it must be said that this change, as stated above, is wrought by Christ's words which are spoken by the priest, so that the last instant of pronouncing the words is the first instant in which Christ's body is in the sacrament; and that the substance of the bread is there during the whole preceding time. Of this time no instant is to be taken as proximately preceding the last one, because time is not made up of successive instants, as is proved in Phys. vi. And therefore a first instant can be assigned in which Christ's body is present; but a last instant cannot be assigned in which the substance of bread is there, but a last time can be assigned. And the same holds good in natural changes, as is evident from the Philosopher (Phys. viii).
Reply to Objection 2. In instantaneous changes a thing is "in becoming," and is "in being" simultaneously; just as becoming illuminated and to be actually illuminated are simultaneous: for in such, a thing is said to be "in being" according as it now is; but to be "in becoming," according as it was not before.
Reply to Objection 3. As stated above (ad 1), this change comes about in the last instant of the pronouncing of the words. for then the meaning of the words is finished, which meaning is efficacious in the forms of the sacraments. And therefore it does not follow that this change is successive.
Source: Summa Theologica (New Advent)