Then I said to you, “do not be shocked, nor fear them”
The LORD your GOD WHO goes before you will HIMSELF fight on your behalf
Just as HE did for you in Egypt before your eyes
And in the wilderness where you saw how The LORD your GOD carried you, just as a man carries his son
In all the way which you have walked until you came to this place
But for all this, you did not trust The LORD your GOD.
Deuteronomy 1:29-32

Background
Moses spoke the aforementioned words to the children of the nation of Israel whom our Beloved LORD GOD had brought out of Egypt (Num 14:31). Almost forty years had passed (Deu 1:3) and the children of the unbelieving nation of Israel were once again standing at the border of the promise land (Deu 1:5). Moses knowing he would not enter the promise land with the children of Israel (Num 20:1-12, Deu 1:37, Deu 3:23-29), under our Beloved LORD GOD’s instruction, prepared them for entry into the promise land by reminding them of their history (Deu chptrs 1-3); renewed the call to obedience while emphasizing that their obedience was tied to their success (Deu 4:1-40); taught them again the Commandments and Law (Deu 4:41-49, Deu chptrs 5-30); encouraged them to trust in The LORD and take the promise land (Deu 31:4-29); appointed Joshua their leader (Deu 31:1-3); and blessed them (Deu 33:1-29).

What does all this have to do with a lethal condition of the heart? An unbelieving heart is one of the conditions that lead to spiritual death. A heart that does not believe in our Beloved LORD GOD (Num 14:11, Exd 32:1-6), nor trust HIS WORDS (Psa 106:24, Heb 9:7-19), is a sin filled heart that continually turns away from HIM (Isa 1:4, Isa 59:12-13, Jer 17:13, 2 Tim 4:3-4); a heart that denies HIM (Pro 30:9, Job 31:24-28, Eze 8:12, Tit 1:16); and a heart that will not be able to see evidence of HIS continual provision and protection (2 Chr 32:24-25, Psa 106:7, Jer 17:5-6). We see an unbelieving heart in full demonstration by the actions of Israel in Deu 1:30-32 and Psa 106:6-48. Despite all the miracles our Beloved LORD GOD performed on behalf of the unbelieving Israelites within Egypt and even in the Promised Land, they continued to go from bad to worse. They behaved wickedly (Psa 106:6), had no understanding of GOD’s wonders (Psa 106:7), did not remember HIS kindness (Psa 106:7), quickly forgot HIS works (Psa 106:13), did not wait for HIS counsel (Psa 106:13), and finally forgot GOD HIMSELF and all the wonders HE performed (Psa 106:21-22).

What does it mean to believe? To truly believe in our Beloved LORD GOD and to really trust in HIS WORDS is more than a mental belief. It is a trust in GOD that is so complete and a reliance upon HIS WORD so absolute, that one’s confidence does not waver regardless of circumstances. It is a belief or trust, which can only come through intimately knowing our Beloved LORD JESUS CHRIST. And over time and through every situation HE develops this level of belief and trust in those who love HIM (Heb 12:2).

Is there any hope for an unbelieving heart? Yes the answer is found in Matthew 7:7-11

Yet, with respect to the promise of GOD
He did not waver in unbelief but grew strong in faith
Giving glory to GOD, and being fully assured that what GOD had promised, HE was able also to perform
Therefore IT WAS ALSO CREDITED TO HIM AS RIGHTEOUSNESS
Romans 4:20-22