This scene refers to the verse in the Old Testament, Book of Numbers, in which Moses sends 12 spies to Canaan. God had decreed that the Israelites would have a place to call their own, the Promised Land, which was believed to be Canaan. Moses, sending spies from each of the twelve tribes, wished to know the lay of the land, the strength of its people. Going forth, they spied for forty days. Upon returning they gave Moses their report. Ten of the twelve spies decreed that the Israelites could not take possession of the land, that the people were too strong and the cities too fortified. Only two spies, Joshua and Caleb, disagreed and urged the people to fight for the Promised Land. They were not listened to and the people despaired, not trusting in their God. Those that had not believed were struck down with plague and the Israelites were forced to wander the wilderness for 40 years in penance.
The two men depicted are thus Joshua, who was to eventually lead the Israelites to the promised land, and Caleb. They are shown holding a large cluster of grapes, to testify to the goodness and fertility of the land they spied upon.