What was the spiritual condition of the generation of Israel that was old enough to respond?
The generation of Israel old enough to respond was marked by total unbelief. Though they had heard, they provoked God and hardened their hearts, and their carcasses fell in the wilderness. Only a few escaped that judgment in their unbelief: Joshua and Caleb, and Moses and Aaron.
Les explains that the nation old enough to respond was, for the most part, totally in unbelief, and he says it is hard to comprehend. They heard God's voice yet provoked Him in the provocation, and their carcasses fell in the wilderness over the 40 years. Only a few survived that judgment in their unbelief—Joshua and Caleb, and Moses and Aaron. This follows the warning not to harden hearts, and the call to hold confidence steadfast to the end.
For some, when they had heard, did provoke: howbeit not all that came out of Egypt by Moses.