What is the significance of Numbers 14:12–20?

In response to Israel’s bickering the Lord says “I will strike them with pestilence and disinherit them, and I will make of you [Moses] a nation greater and mightier than they” (vs. 12). Moses asks the Lord to forgive the people, and the Lord eventually responds, “I do forgive, just as you have asked” (vs. 20).

Unless the intention the Lord declared to Moses in verse 12 was insincere, we must conclude that he did not at that point intend on forgiving the Israelites. It cannot have been certain at that time (let alone from all eternity) that God would forgive the Israelites. Hence, it seems that either the Lord is insincere, or the classical view of divine foreknowledge is mistaken.