Commentators unanimously identify the core sin not as mere complaining, but as profound unbelief. After witnessing God's miraculous provision, Israel's doubt in His ability to provide further was seen as "wanton and wicked" (Spurgeon). Calvin explains that faith is the root of piety, while unbelief inevitably leads to murmuring and rebellion. God's anger was a response to this deep-seated distrust in His power and salvation.