Objection 1. It seems that the will of God imposes necessity on the things willed. For Augustine says (Enchiridion 103): "No one is saved, except whom God has willed to be saved. He must therefore be asked to will it; for if He wills it, it must necessarily be."
Objection 2. Further, every cause that cannot be hindered, produces its effect necessarily, because, as the Philosopher says (Phys. ii, 84) "Nature always works in the same way, if there is nothing to hinder it." But the will of God cannot be hindered. For the Apostle says (Romans 9:19): "Who resisteth His will?" Therefore the will of God imposes necessity on the things willed.
Objection 3. Further, whatever is necessary by its antecedent cause is necessary absolutely; it is thus necessary that animals should die, being compounded of contrary elements. Now things created by God are related to the divine will as to an antecedent cause, whereby they have necessity. For the conditional statement is true that if God wills a thing, it comes to pass; and every true conditional statement is necessary. It follows therefore that all that God wills is necessary absolutely.
On the contrary, All good things that exist God wills to be. If therefore His will imposes necessity on things willed, it follows that all good happens of necessity; and thus there is an end of free will, counsel, and all other such things.
I answer that, The divine will imposes necessity on some things willed but not on all. The reason of this some have chosen to assign to intermediate causes, holding that what God produces by necessary causes is necessary; and what He produces by contingent causes contingent. This does not seem to be a sufficient explanation, for two reasons.
First, because the effect of a first cause is contingent on account of the secondary cause, from the fact that the effect of the first cause is hindered by deficiency in the second cause, as the sun's power is hindered by a defect in the plant. But no defect of a secondary cause can hinder God's will from producing its effect.
Secondly, because if the distinction between the contingent and the necessary is to be referred only to secondary causes, this must be independent of the divine intention and will; which is inadmissible. It is better therefore to say that this happens on account of the efficacy of the divine will. For when a cause is efficacious to act, the effect follows upon the cause, not only as to the thing done, but also as to its manner of being done or of being. Thus from defect of active power in the seed it may happen that a child is born unlike its father in accidental points, that belong to its manner of being. Since then the divine will is perfectly efficacious, it follows not only that things are done, which God wills to be done, but also that they are done in the way that He wills. Now God wills some things to be done necessarily, some contingently, to the right ordering of things, for the building up of the universe. Therefore to some effects He has attached necessary causes, that cannot fail; but to others defectible and contingent causes, from which arise contingent effects. Hence it is not because the proximate causes are contingent that the effects willed by God happen contingently, but because God prepared contingent causes for them, it being His will that they should happen contingently.
Reply to Objection 1. By the words of Augustine we must understand a necessity in things willed by God that is not absolute, but conditional. For the conditional statement that if God wills a thing it must necessarily be, is necessarily true.
Reply to Objection 2. From the very fact that nothing resists the divine will, it follows that not only those things happen that God wills to happen, but that they happen necessarily or contingently according to His will.
Reply to Objection 3. Consequents have necessity from their antecedents according to the mode of the antecedents. Hence things effected by the divine will have that kind of necessity that God wills them to have, either absolute or conditional. Not all things, therefore, are absolute necessities.
Source: Summa Theologica (New Advent)