The book of Hebrews can be viewed as built around multiple warnings to continue in the faith and against falling away from the faith (2:1-4; 3:7-4:13; 5:11-6:12; 10:26-39; 12:15-17; 12:25-29). Just as in the first warning to hear the message of God declared in his Son by attestation of the Holy Spirit; the author of Hebrews forms his second warning in a Trinitarian manner.
Therefore, as the Holy Spirit says, “Today, if you hear his voice, do not harden your hearts as in the rebellion, on the day of testing in the wilderness, where your fathers put me to the test and saw my works for forty years. Therefore I was provoked with that generation, and said, ‘They always go astray in their heart; they have not known my ways.’ As I swore in my wrath, ‘They shall not enter my rest.'” Take care, brothers, lest there be in any of you an evil, unbelieving heart, leading you to fall away from the living God. But exhort one another every day, as long as it is called “today,” that none of you may be hardened by the deceitfulness of sin. For we have come to share in Christ, if indeed we hold our original confidence firm to the end. (Hebrews 3:7-14)
Whereas the first warning in Hebrews was to pay attention to the message delivered by the Trinity, this second warning begins to explain how we know we are paying attention to that message. The teaching is accurately summarized by the maxim, be doers of the word and not hearers only.
The person who will inherit the rest of God is the one that perseveres until the end. This emphasis of continuing “to the end” is directly from Jesus: “But the one who endures to the end will be saved” (Matt. 10:22 & 24:13). How does one continue to the end? The way to continue to the end is to exhort fellow believers daily. While Hebrews 10:25 is often used in support of faithful church attendance, it should not be read apart from Hebrews 3:13 and the instruction to practice exhortation “every day.” Unfortunately we separate these two verses from one another and from the centuries-long practice of the early church.
The sure sign of a believing heart is not the exuberant youth caught up in the “original confidence” of trusting Christ. The sure sign of a believing heart is the 80 year old widow who has remained faithful in tangibly supporting the ministry of her church.
What does it mean to fall away from God? What does it mean to lose fellowship with Christ? Whatever it may or may not mean, the surest indication that one has done so is that he stops fellowshipping with God’s people. That is why the Holy Spirit says, “Today, if you hear his voice, do not harden your hearts.”
The larger context of this passage is also important for formulating a proper understanding of the nature of Scripture. Hebrews 3:7 unmistakably indicates a super-natural source for Scripture: the words of Psalm 95 are the words of the Holy Spirit. In the following verses, the author Hebrews continues to explain Psalm 95 and in 4:7 asserts that the words of the Psalm came “through David.”
This does not answer all of our questions about the nature of Scripture, indeed it perhaps arouses even more. Even so, the author of Hebrews presents Scripture (at least Psalm 95) as the product of the Holy Spirit speaking through men.