Jamieson, Fausset and Brown on Exodus 16:4

4. Then said the Lord unto Moses--Though the outbreak was immediately against the human leaders, it was indirectly against God: yet mark His patience, and how graciously He promised to redress the grievance.

I will rain bread from heaven--Israel, a type of the Church which is from above, and being under the conduct, government, and laws of heaven, received their food from heaven also (Psa 78:24).

that I may prove them, whether they will walk in my law, or no--The grand object of their being led into the wilderness was that they might receive a religious training directly under the eye of God; and the first lesson taught them was a constant dependence on God for their daily nourishment.