Korach, Moses' cousin, stages a rebellion against Moses' authority and Aaron's appointment as high priest. There was a crowd of people whom Korach's had drawn to his side. Moses tried to convince them to abandon Korach's mutiny, but they refused to back down. Moses warned them that G-d would open up a pit in the ground that would swallow them up.
Numbers 16:17-8
Moses knew that all he had done was in full accordance with G-d's will, and that Korach's arguments were faulty and misguided. Moses could have therefore chosen not to react to Korach, opting instead to let events unfold as they might, confident that in the end Korach would be proven wrong and his plot would fail.
But instead, Moses tried to convince Korach of his error, and when that failed, he devised a test whereby Korach's error could be demonstrated. Moses knew that when people are mistaken everything possible must be done to correct them.
G-d has promised us that He will eventually bring everyone back to observance of the Torah's commandments, no matter how far they have strayed. Nonetheless, we must, as did Moses, do all that we can to ensure that everyone manifest their relationship with G-d now, rather than rely on G-d's promise and allow them to be estranged from Him (consciously or not) for even one extra moment.
—from Daily Wisdom 3, P. 304