To murmur is to complain and grumble, resentfully and subversively, under our breath, in a rebellious, discontented manner. Murmuring against God is a serious sin; we should take a lesson from Israel on this one (1Co 10:10), avoid this wickedness and root it out of our lives.
Discontentment and resentment come from thinking we deserve better, so murmuring against God is accusing Him of treating us inappropriately.
Perhaps we’ve made some poor decisions and we’re now suffering for it. Is that God’s fault? It’s true He could have stopped us, but He didn’t. Was that unfair? Is God obligated to restrain us from our own foolishness? Is it right to be resentful? (Pr 19:3)
Or maybe someone sinned against us and we’re now suffering as a consequence. Is that God’s fault? It’s true He could have stopped them, but He didn’t. Was that unfair? Is God obligated to restrain others from their malice or negligence? Is it right to be resentful? (Ro 11:36)
Or we might be sick or in pain and we’re now suffering in the midst of it. Is that God’s fault? It’s true He could heal us and stop the pain, but He hasn’t, and we don’t see any rhyme or reason in it. Is that unfair? Is it unloving? Is God obligated to make our lives pain-free and easy? Does He need to explain everything to us? Is it right to be resentful? (1Pe 1:6-7)
If we harbor resentment toward God, if we aren’t giving thanks to Him in our circumstances (Ep 5:20), if we lose hope (1Co 13:7), we’re accusing Him of being unjust; we’re denying His goodness, fainting in the day of adversity. (Pr 24:10)
The remedy here is to humble ourselves; murmuring can only be grounded in pride, thinking we deserve better. If we’ve been foolish, let’s own it and seek wisdom. (Ja 1:5) If we’re suffering otherwise, let’s trust God’s plan for good in it (Ro 8:28), hope in Him (Ro 8:24) and wait on Him. (Ps 27:14) We can glorify God in all His ways. (Re 15:4)