1)
What was their lack of belief in Hashem?
1.
Radak: That He can give to them meat, and bring them into Eretz Kena'an.
2.
Malbim: They thought that His power is limited, and therefore they doubted if He can save them [from the Kena'anim] to inherit the land.
2)
Why does it say "v'Lo Vatchu bi'Yshu'aso"?
1.
Radak: He saved them several times (and even so, they did not rely on Him).
2.
Malbim: Refer to 78:22:1:2.
