Not left to mystery as to why

Even though Dekker did not give free will as the answer to the question, how is it that not all men are saved? the answer that he did give was a step in the wrong direction.

Let us . . . [leave] the unexplainable where it belongs - in the infinite mystery of the heart of Him who is himself love. On the side of divine sovereignty, then, there is mystery. On the side of human responsibility, however, there is no mystery at all. The answer is plainly a matter of unbelief.

It is true that, concerning man's responsibility, the reason many perish is their own unbelief. The fault is man's own. But it is not true, regarding God's sovereignty, that one can refer only to the mystery. The scriptures teach that the explanation why not all men are saved is that God has eternally reprobated some men, eternally decreeing that they perish in their sins, and that God, according to his decree does not bestow faith upon them but hardens them in sin. In Romans 9, in explanation of the fact that some in Israel were saved and others were not (vv. 6-8), Paul plainly teaches that God loved and chose some but hated and reprobated others (vv. 11-13). According to this double predestination, God has mercy on some but hardens the others (v. 18). The reason there are 'vessels of mercy . . . afore prepared unto glory' (v. 23) is that 'the potter [has] power over the clay . . . to make one vessel unto honour' (v. 21). The reason there are 'vessels of wrath fitted to destruction' (v. 22) is that 'the potter' has power also 'to make . . . another [vessel] unto dishonour' (v. 21). - David J. Engelsma, Hyper-Calvinism and the Call of the Gospel