1. An Evil Heart of Unbelief
What is it?
How is it caused?
How can it be avoided?

2. Hebrews 3:12-13
Beware, brethren, lest there be in any of you an evil heart of unbelief in departing from the living God; but exhort one another daily, while it is called "Today," lest any of you be hardened through the deceitfulness of sin. 

3. Background
The Hebrew Christians had been facing trials and persecutions because of their religion.
Some had fallen away.
The epistle to the Hebrews was written to encourage the rest not to develop “an evil heart of unbelief in departing from the living God.”
But what is an evil heart of unbelief?
How is an evil heart of unbelief caused?
How can an evil heart of unbelief be avoided?

4. What Is An Evil Heart?
It is not man’s physical heart, but his spiritual heart that can become evil.

5. What Is An Evil Heart?
“By an easy transition the word [heart] came to stand for man's entire mental and moral activity, both the rational and the emotional elements” (Vine's p. 297).

6. What Is An Evil Heart?
It is his intellectual, emotional, ethical and volitional natures combined.

7. What Is An Evil Heart?
It is the seedbed for all things good & evil in man.

8. Matthew 12:35; 15:19
A good man out of the good treasure of his heart brings forth good things, and an evil man out of the evil treasure brings forth evil things. For out of the heart proceed evil thoughts, murders, adulteries, fornications, thefts, false witness, blasphemies. 

9. What Is An Evil Heart?
An evil heart contains mostly evil treasure and is therefore set on evil.

10. What Is An Evil Heart of Unbelief?
There are two kinds of unbelief.
Doctrinal - Not believing
Practical - Not seeking

11. Hebrews 11:6
But without faith it is impossible to please Him, for he who comes to God must believe that He is, and that He is a rewarder of those who diligently seek Him. 

12. What Is An Evil Heart of Unbelief?
There are two kinds of unbelief.
Doctrinal - Not believing
Practical - Not seeking

13. What Is An Evil Heart of Unbelief?
Unbelief in the immediate context.

14. Hebrews 3:7-11
Therefore, as the Holy Spirit says: "Today, if you will hear His voice, Do not harden your hearts as in the rebellion, In the day of trial in the wilderness, Where your fathers tested Me, tried Me, And saw My works forty years. Therefore I was angry with that generation, And said, 'They always go astray in their heart, And they have not known My ways.' So I swore in My wrath, 'They shall not enter My rest.' "

15. Is this doctrinal or practical unbelief?
Hebrews 3:16-19
For who, having heard, rebelled? Indeed, was it not all who came out of Egypt, led by Moses? Now with whom was He angry forty years? Was it not with those who sinned, whose corpses fell in the wilderness? And to whom did He swear that they would not enter His rest, but to those who did not obey? So we see that they could not enter in because of unbelief.
Is this doctrinal or practical unbelief?

16. "Obstinacy, obstinate rejection Clearly, this is practical unbelief.
Hebrews 4:6-7, 11
Since therefore it remains that some must enter it, and those to whom it was first preached did not enter because of disobedience, again He designates a certain day, saying in David, "Today," after such a long time, as it has been said: "Today, if you will hear His voice, Do not harden your hearts."
Let us therefore be diligent to enter that rest, lest anyone fall according to the same example of disobedience. 
"Obstinacy, obstinate rejection
of the will of God."
Clearly, this is practical unbelief.