Objection 1. It seems that the predestined are not chosen by God. For Dionysius says (Div. Nom. iv, 1) that as the corporeal sun sends his rays upon all without selection, so does God His goodness. But the goodness of God is communicated to some in an especial manner through a participation of grace and glory. Therefore God without any selection communicates His grace and glory; and this belongs to predestination.
Objection 2. Further, election is of things that exist. But predestination from all eternity is also of things which do not exist. Therefore, some are predestined without election.
Objection 3. Further, election implies some discrimination. Now God "wills all men to be saved" (1 Timothy 2:4). Therefore, predestination which ordains men towards eternal salvation, is without election.
On the contrary, It is said (Ephesians 1:4): "He chose us in Him before the foundation of the world."
I answer that, Predestination presupposes election in the order of reason; and election presupposes love. The reason of this is that predestination, as stated above (Article 1), is a part of providence. Now providence, as also prudence, is the plan existing in the intellect directing the ordering of some things towards an end; as was proved above (Question 22, Article 2). But nothing is directed towards an end unless the will for that end already exists. Whence the predestination of some to eternal salvation presupposes, in the order of reason, that God wills their salvation; and to this belong both election and love:--love, inasmuch as He wills them this particular good of eternal salvation; since to love is to wish well to anyone, as stated above (20, 2, 3):--election, inasmuch as He wills this good to some in preference to others; since He reprobates some, as stated above (Article 3). Election and love, however, are differently ordered in God, and in ourselves: because in us the will in loving does not cause good, but we are incited to love by the good which already exists; and therefore we choose someone to love, and so election in us precedes love. In God, however, it is the reverse. For His will, by which in loving He wishes good to someone, is the cause of that good possessed by some in preference to others. Thus it is clear that love precedes election in the order of reason, and election precedes predestination. Whence all the predestinate are objects of election and love.
Reply to Objection 1. If the communication of the divine goodness in general be considered, God communicates His goodness without election; inasmuch as there is nothing which does not in some way share in His goodness, as we said above (Question 6, Article 4). But if we consider the communication of this or that particular good, He does not allot it without election; since He gives certain goods to some men, which He does not give to others. Thus in the conferring of grace and glory election is implied.
Reply to Objection 2. When the will of the person choosing is incited to make a choice by the good already pre-existing in the object chosen, the choice must needs be of those things which already exist, as happens in our choice. In God it is otherwise; as was said above (Question 20, Article 2). Thus, as Augustine says (De Verb. Ap. Serm. 11): "Those are chosen by God, who do not exist; yet He does not err in His choice."
Reply to Objection 3. God wills all men to be saved by His antecedent will, which is to will not simply but relatively; and not by His consequent will, which is to will simply.
Source: Summa Theologica (New Advent)