Hebrews 3:9-12

Heb. 3:9-12. When your fathers tempted me, proved me, and saw my works forty years. Wherefore I was grieved with that generation, and said, They do always err in (their) heart; and they have not known my ways. So I swore in my wrath, They shall not enter into my rest. Take heed, brethren, lest there be in any of you an evil heart of unbelief, in departing from the living God.

Blank Bible:

[I]n departing and having respect to that which he quotes from the Psalmist in the 10th verse. They do always err in their hearts, etc.

Edwards cites this verse showing the essential identity of trusting in the Old Testament and faith in the New Testament in Concerning Faith:

§56. That believing in the New Testament, is much the same as trusting, in the Old, is confirmed by comparing Jeremiah 17:5, "Cursed is the man that trusteth in man, and maketh flesh his arm, and whose heart departeth from the Lord;" verse 7, "Blessed is the man that trusteth in the Lord, whose hope the Lord is," - with Hebrews 3:12, "Take heed, brethren, lest there be in any of you an evil heart of unbelief, in departing from the living God." It also is confirmed by this, that trusting in God, and hoping in him, are used in the Old Testament as expressions of the same import. So hope is often in the New Testament used to signify the same thing that, in other places is signified by faith. Romans 15:12; Romans 15:13. And again, Esaias saith, There shall be a root of Jesse, and he that shall rise to reign over the Gentiles, in him shall the Gentiles trust." - "Now the God of peace fill you with all joy and peace in believing, that ye may abound in hope through the power of the Holy Ghost." Compare Daniel 6:23 and Hebrews 11:33; Hebrews 11:34.

Later in the same treatise Edwards explains why it is faith which is said to justify.

Why are we not said to be justified by light as well as by faith? For is there not the same virtue in seeing things visible, as in believing things credible? Tell me then, what is faith, that it should raise men above the level of mortality, and make men become like the angels of heaven? - But further, if it be only an act of the understanding formed upon due reasons, how comes it to be described in Scripture as having its seat in the heart? The apostle in the text (Hebrews 3:12), cautions against an evil heart of unbelief; and the same notion prevails throughout the books of Scripture, and is as early as our Saviour's first preaching. Faith, which is the principle of the gospel, respects the promises and declaration of God, and includes a sure trust and reliance on him for the performance. Beyond this, there is no further act of faith. We are not taught to believe this, in order to our believing something else; but here, faith has its full completion, and leads immediately to the practice of virtue and holiness. For this end was the Son of God revealed, to make known the mind and will of the Father, to declare his mercy and pardon, and to confirm the promises of eternal life to mankind. He that believes and accepts this deliverance from the bondage of sin, and through patience and perseverance in well doing, waits for the blessed hope of immortality; who passes through the world as a stranger and pilgrim, looking to another country, and a city whose builder is God; this is he whose faith shall receive the promise, whose confidence shall have great recompense of reward."

The grave need of self-examination mentioned in verse 12 comes in for expected notice:

On the account of this deceitfulness of sin, and because we have so much sin dwelling in our hearts, it is a difficult thing to pass a true judgment on our own ways and practices. On this account we should make diligent search, and be much concerned to know whether there be not some wicked way in us. Hebrews 3:12; Hebrews 3:13, "Take heed, 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 to-day, lest any of you be hardened through the deceitfulness of sin."

Heb. 3:13