John Calvin on Numbers 16:12

12. And Moses sent to call Dathan and Abiram. He desired, in this way, if it might be, by his holy admonitions, to withhold them from that destruction, on which they were rushing. Therefore he ceased not to provide for their welfare, though he had thus far experienced that they were altogether in a desperate state. Herein he presented a likeness of the loving-kindness of God, by whose Spirit he was directed; not only because he was unwilling to pass sentence without hearing the cause, but also because he endeavored to bring them to repentance, that they might not willfully destroy themselves. Nevertheless it came to pass at this time, as also often afterwards, that not only was the earnestness of the Prophet, with respect to these unbelievers, throw away, but that it hardened them more and more. For we know what was said by Isaiah;

“Make the heart of this people fat, and make their ears heavy, and shut their eyes: lest they see with their eyes, and hear with their ears, and understand with hearts, and convert, and be healed.” (Isaiah 6:10.)

Thus does it please God to discover the wickedness of the reprobate, in order that they may be rendered the more inexcusable.