Why is complaining against God a serious issue?

In the sermon, it's emphasized that complaining is fundamentally against God, as seen in the actions of the Israelites after their deliverance. Their murmuring was a direct affront to the faithfulness of God who had just delivered them. This attitude reveals a heart filled with unbelief, failing to trust that God, who did the impossible, would also provide for their needs. The root of all complaining is distrust, and instead of murmuring, the preacher encourages us to entreat the Lord in times of need, reinforcing the importance of prayer over complaint.