What sin did God point to when Israel was kept from entering His rest—the golden calf or unbelief?
God pointed to Israel's unbelief, not the golden calf. At Kadesh Barnea they refused to enter Canaan because they did not believe they could take it. Scripture says God did not remind them of their immorality; He reminded them of their unbelief.
Les explains that Hebrews 3 rehearses the event at Kadesh Barnea. The people heard but provoked God, and their carcasses fell in the wilderness; God swore they would not enter His rest, which Canaan would have been. One might think the sin was worshiping the golden calf, since a few months earlier Aaron made the calf while Moses was on the mountain, and gross immorality followed. But several months later, when they were ready to move into Kadesh Barnea, they turned around in unbelief and said, "We can't take it." God did not remind them of their immorality; He reminded them of their unbelief.
While it is said, To day if ye will hear his voice, harden not your hearts, as in the provocation.