We must begin by understanding that there are two distinct aspects to reprobation. It is important to keep these distinctions in mind all the time.
Preterition comes from the Latin, and means "to pass by." It is flip side to election and has nothing whatever to do with the merits or demerits of the person passed by. If men are elected without reference to their works, then this necessitates the truth that other men are not elected without regard to their works.
Condemnation is for sin. The reason men fall under the judgment of God is because they are sinners. They do not fall under His judgment because they are "not elect."
Consider a terminal cancer ward. Suppose someone came into the ward and gave a cure to half of the patients. They recovered, and those who did not receive the medicine died. Now, what did they die of? Cancer? Or of "not taking medicine"? It is the same here. Men are judged for their sins, and not for their non-elect status.
Preterition is therefore unconditional (just like election), and condemnation is conditional (it is a wage).
Unbelief and Sin are Controlled by God
Before we seek to defend any doctrine we must first ensure that the doctrine is a scriptural one. This is especially the case with a doctrine as volatile as this one.
God hardens hearts: Joshua 11:20; Dt. 2:30; Ex. 4:21
God controls wicked men: Job 1:13-15, 17, 21; Is. 10:5-6; Judges 9:22-23; 1 Sam. 2:25; 2 Sam. 12:11-12; Ps. 105:24-25; Prov. 16:4; 2 Thess. 2:11-12
But What About with Regard to Salvation?
The Bible is no less clear here.
Consider what it says here:
Therefore, to you who believe, He is precious; but to those who are disobedient, "The stone which the builders rejected has become the chief cornerstone," and "A stone of stumbling and a rock of offense." They stumble, being disobedient to the word, to which they also were appointed. But you are a chosen generation, a royal priesthood, a holy nation, His own special people, that you may proclaim the praises of Him who called you out of darkness into His marvelous light . . . (1 Pet. 2:7-9).
Also in another place:
For God did not appoint us to wrath, but to obtain salvation through our Lord Jesus Christ . . . (1 Thess. 5:9)
And in one more place:
For certain men have crept in unnoticed, who long ago were marked out for this condemnation, ungodly men, who turn the grace of our God into licentiousness and deny the only Lord God and our Lord Jesus Christ (Jude 4)
And all application must be made with fear and trembling.