Reprobation is the eternal, sovereign, and righteous decree of God whereby He passes by some of the fallen human race, leaving them in their sin and justly punishing them for their iniquity to the praise of His glorious justice. While the Reformed tradition affirms this doctrine, it strictly maintains that God is never the author of sin, as the reprobate willfully plunge themselves into their own ruin.
Reprobation is the necessary counterpart to the doctrine of election. As stated in the 1689 LBCF Ch.3 §3, while some are predestinated to eternal life through Jesus Christ, others are 'left to act in their sin to their just condemnation, to the praise of his glorious justice.' This does not suggest that God created innocent people to destroy them; rather, He leaves those who have willfully fallen in Adam to follow their own desires, ultimately rendering the verdict of judgment upon their unbelief and their sins (Canons of Dort Head 1:15). God is not the author of sin—that would be blasphemy—but He is the righteous Judge who manifests His glory both in His mercy toward the elect and His justice toward the reprobate.
It is easy for the human mind to rebel against this doctrine, as if God were being cruel or capricious. However, Scripture warns us against such arrogance. The Apostle Paul asks, 'Nay but, O man, who art thou that repliest against God? Shall the thing formed say to him that formed it, Why didst thou make me thus? Or hath not the potter a right over the clay...' (Romans 9:20-21). We are all from the same 'corrupt mass' of humanity; the miracle is not that some are justly condemned, but that any are saved at all. At the last day, the purpose of this decree will be clear: 'The end of God's appointing this day, is for the manifestation of the glory of his mercy, in the eternal salvation of the elect; and of his justice, in the eternal damnation of the reprobate, who are wicked and disobedient' (1689 LBCF Ch.32 §2). We find our comfort not in prying into these secret decrees, but by looking to Christ, in whom we are accepted and loved more than we could ever hope.