Lack of faith in God and his gospel.

He did not many mighty works there because of their unbelief, Matt. 13: 58. Because of their unbelief Jesus' disciples could not cast out a devil, Matt. 17: 14-21. Help thou mine unbelief, Mark 9: 23-24. Jesus upbraided his Apostles with their unbelief and hardness of heart, Mark 16: 14. Shall unbelief make the faith of God without effect? Rom. 3: 3. It is better that one man should perish than that a nation should dwindle and perish in unbelief, 1 Ne. 4: 13. When the time cometh that they dwindle in unbelief, he will cause them to be scattered and smitten, 2 Ne. 1: 10-11 (D&C 3: 18). Because of their unbelief they could not understand the word of God, Mosiah 26: 1-5. I could not show unto the Jews so great miracles, because of their unbelief, 3 Ne. 19: 35. Your minds in times past have been darkened because of unbelief, D&C 84: 54-58.