The church may not officially say too much on this mystery but the Scriptures and Church Fathers sure do. Let's start with the Scriptures.
The Scriptures teach that God made some "vessel's unto dishonor," and "endures with longsuffering vessels of wrath fitted to destruction" so that "He might make known the riches of His glory to us, vessels of mercy." In other words God is showing us that we are no different than vessels of wrath and except for God's Sovereign grace on us we would also be destroyed, this revealed truth should bring us great joy for His love for and choice of us for salvation.
ROM 9:21 Hath not the potter power over the clay, of the same lump to make one vessel unto honour, and another unto dishonour?
ROM 9:22 What if God, willing to show his wrath, and to make his power known, endured with much longsuffering the vessels of wrath fitted to destruction:
ROM 9:23 And that he might make known the riches of his glory on the vessels of mercy, which he had afore prepared unto glory,
Augustine wrote the same interpretation of these verses:
"We must thoroughly examine the Apostle's purpose as to why, in order to emphasize grace, [God] did not choose that the one of whom it was said Jacob I loved, should glory except in the Lord [Mal. 1:3, 1 Co. 1:31]. [Jacob and Esau] were from the same father and the same mother, by one act of intercourse - - and before they had done anything good or evil, God loved the one and hated the other. This was so that Jacob would realize that he was from the lump of original iniquity when he saw that his brother, with whom he had a common origin, in justice deserved to be condemned, and that he could be distinguished only through grace. For, [Paul] says, even before they were born or had done anything good or bad -- in order that God's purpose of election might continue, not by works but by His call -- [Rebecca] was told, 'The elder shall serve the younger.'" (The
excerpts from Augustine on the Epistles of Paul compiled by the English monk, Bede the Venerable (c. 673-735) p.84-87)
Aquinas wrote the following:
"I answer that, God does reprobate some. For it was said above (Ar1) that predestination is a part of providence. To providence, however, it belongs to permit certain defects in those things which are subject to providence, as was said above (Q22:Ar2). Thus, as men are ordained to eternal life through the providence of God, it likewise is part of that providence to permit some to fall away from that end; this is called reprobation. Thus, as predestination is a part of providence, in regard to those ordained to eternal salvation, so reprobation is a part of providence in regard to those who turn aside from that end. Hence reprobation implies not only foreknowledge, but also something more, as does providence, as was said above (Q22:Ar1). Therefore, as predestination includes the will to confer grace and glory; so also reprobation includes the will to permit a person to fall into sin, and to impose the punishment of damnation on account of that sin." (The Summa Theologica by Thomas Aquinas, Question 23: Of the Predestination).
