What does the Bible say about complaining against God?

Complaining against God is seen as a manifestation of unbelief, as highlighted in Exodus 15 when the Israelites, after witnessing God's miraculous deliverance, quickly turned to murmur against Him. The underlying issue is not merely their circumstances, but their hearts, which are filled with bitterness. The scriptures urge believers to trust in God's perfect planning and providence, as recalling His former faithfulness fosters gratitude instead of discontent. This root of unbelief is what often leads to murmuring, rather than a spirit of thankfulness.