John Calvin Commentary Numbers 16:12

"And Moses sent to call Dathan and Abiram, the sons of Eliab; and they said, We will not come up:" — Numbers 16:12 (ASV)

And Moses sent to call Dathan and Abiram. He desired, in this way, if possible, by his holy admonitions, to keep them from the destruction toward which they were rushing.

Therefore, he did not cease to provide for their welfare, even though he had experienced up to that point that they were in a completely desperate state. In this, he presented a reflection of the loving-kindness of God, by whose Spirit he was directed. He was not only unwilling to pass sentence without hearing their case, but he also strove to bring them to repentance, so that they would not willfully destroy themselves.

Nevertheless, it happened at this time, as often happened afterwards, that not only was the Prophet's earnestness toward these unbelievers wasted, but it also 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 it pleases God to reveal the wickedness of the reprobate, so that they may be rendered even more inexcusable.