We proceed to the sixth article thus:
1. It seems that predestination is not certain. For on Rev.3:11, "hold that fast which thou hast, that no man take thy crown," Augustine says: "no other will take it if one does not lose it." The crown to which one is predestined may therefore be lost as well as won. Hence predestination is not certain.
2. Again, if something is possible, none of its consequences are impossible. Now it is possible for a predestined man, like Peter, to sin and to fall. But if he should, the effect of predestination would be frustrated in consequence. The frustration of the effect of predestination is therefore not impossible. Hence predestination is not certain.
3. Again, what God could have done, that he can do. But God could have omitted to predestine one whom he has predestined, and therefore may not predestine him now. Hence predestination is not certain.
On the other hand: in a gloss on Rom.8:29, "whom he did foreknow, he also did predestinate," Augustine says: "predestination is the foreknowledge and preparation of God's blessings, by which  whosoever will be set free will most certainly be set free" (De Dono Persev.14).
I answer: predestination achieves its effect most certainly and infallibly. But it does not impose necessity of such a kind that its effect is realized through necessity. We said in Art.1 that predestination is part of providence. But the things over which providence rules do not all come about through necessity. Some of them are realized through contingency, in accordance with the condition of the immediate causes which providence has provided for them. The ordinance of providence is nevertheless infallible, in spite of this. Now the ordinance of predestination is infallible in the same way. It does not exclude the freedom of the will, but realizes its effects contingently by means of it. What we said concerning the knowledge and will of God (Q.14, Art.13; Q.19, Art.4) must be understood in this light. They do not preclude contingency in things, even though they are certain and infallible.
On the first point: when we say that a crown belongs to someone, we may mean either of two things. We may mean that he is predestined to it. If we mean this, no one loses his crown. But we may also mean that a crown is due on account of merit acquired through grace, since what we deserve in a sense belongs to us. If we mean this, then anyone may lose his crown through subsequent mortal sin. Another then receives the crown which he has lost, being substituted in his stead, since God does not allow any to fall without putting others in their place. As it is said in Job 34:24: "He shall break in pieces mighty men without number, and set others in their stead." Men are thus set in the place of fallen angels, and Gentiles in the place of Jews. One who is substituted in the state of grace also receives the crown of the fallen in the sense that he rejoices in eternal life in the good which the other has done. For in eternal life everyone will rejoice in the good which has been done, whether by oneself or by another.
On the second point: considered in itself, that he should die in mortal sin is a possibility for one who is predestined. But if it is determined that he actually is predestined, this is not a possibility.
On the third point: as we said in Art.4, predestination involves the divine will. Now the divine will is immutable. That God should will what he has created is therefore necessary, given that he has created it, though it is not necessary absolutely. We are bound to say the same of predestination. If all factors are taken into consideration, we must not say that God might not have predestined one whom he has predestined. We could say, speaking absolutely, that God either might or might not have predestined him. But this does not affect the certainty of predestination.
 Migne "qua." Augustine "quibus'.'