An evil heart of unbelief !

Heb 3:12

12 Take heed, brethren, lest there be in any of you an evil heart of unbelief, in departing from the living God.

By Nature all men have an evil heart of unbelief, characterized by the Apostacy of Adam in the beginning, when by the temptation of satan the heart departed from the Living God, hence all the Sheep in Adam were guilty of this Isa 53:6

6 All we like sheep have gone astray; we have turned every one to his own way; and the Lord hath laid on him the iniquity of us all.

So the going astray, the turning to our own way are the same as that departing from the Living God of Heb 3:12,

And because of that, we begin our lives in the flesh after the likeness of Adam, as a prisoner of Unbelief Rom 11:32

32 For God hath concluded them all in unbelief, that he might have mercy upon all.

So while in the flesh, what we are by nature, the heart we receive by nature, its an evil heart of unbelief, that alienates us from God in permanent unbelief !

The unbelief in the human heart is incurable Jer 17:9

9 The heart is deceitful above all things, and desperately wicked: who can know it?

This heart of unbelief is not curable, it cant change its nature, the only remedy is A New Heart from New Birth !

Ezk 36:26

A new heart also will I give you, and a new spirit will I put within you: and I will take away the stony heart out of your flesh, and I will give you an heart of flesh.

There is no escaping a evil heart of unbelief save by the promise above, and then and only then can we believe, have Faith in God which pleases Him Heb 11:6,

In the flesh however, not born anew, we cannot please God with Faith Rom 8:8

So then they that are in the flesh cannot please God.

Thats because, all we have by nature in the flesh, is an evil heart of unbelief !