In the first four chapters of the book of Hebrews the author has been exhorting his readers to persevere in the faith of the gospel, gathering his encouragements in the form of arguments for: the supremacy of Christ as the final and best revelation of God, Christ in his humanity being perfectly suited to be our Savior, and Christ as the Son and builder of God’s house being greater than Moses the faithful servant in the house. Based on the superiority of the salvation offered by Christ he has exhorted his readers not to harden their hearts, but to strive to enter God’s rest, hammering home the point by repeatedly quoting the Scriptures of the Old Testament, especially Psalm 95, emphasizing that we must hear God’s voice “Today” in order to enter that rest. And then, in the flow of his argument he brings forth this staggering description of God’s word in Scripture:
For the word of God is living and active, sharper than any two-edged sword, piercing to the division of soul and of spirit, of joints and of marrow, and discerning the thoughts and intentions of the heart. And no creature is hidden from his sight, but all are naked and exposed to the eyes of him to whom we must give account. —Hebrews 4:12–13
Before we focus on why the author of Hebrews places this passage here, let’s consider the word of God by looking at other ways Scripture speaks of itself throughout the Bible. First, from Isaiah, in the context of calling people to seek the LORD while he may be found and to call upon him while he is near, to return to him that he may have compassion and abundantly pardon:
so shall my word be that goes out from my mouth;
it shall not return to me empty,
but it shall accomplish that which I purpose,
and shall succeed in the thing for which I sent it. —Isaiah 55:11
God’s purpose is to bless his people, and his word is not idle, but has power to successfully accomplish the purpose for which he sends it. For believers, this purpose is life-giving:
. . . you have been born again, not of perishable seed but of imperishable, through the living and abiding word of God; for
“All flesh is like grass
and all its glory like the flower of grass.
The grass withers,
and the flower falls,
but the word of the Lord remains forever.”
And this word is the good news that was preached to you. —1 Peter 1:23–25
By the life-giving and abiding word of God believers are born again. In this new birth, the work of the word in our hearts and minds is only getting started. For the word is the instrument by which we then grow up in our sanctification:
. . . from childhood you have been acquainted with the sacred writings, which are able to make you wise for salvation through faith in Christ Jesus. All Scripture is breathed out by God and profitable for teaching, for reproof, for correction, and for training in righteousness, that the man of God may be complete, equipped for every good work. — 2 Timothy 3:15–17 (in the context of perseverance in the faith)
The word of God is the very breath of God, able to make us wise. It’s not merely useful but we gain when we go to the word for teaching, reproof, correction, and training in righteousness. The word surpasses all worldly methods for completing and equipping us for those works which God has prepared us to do (Eph. 2:10). Moreover, as the breath of God:
In the beginning was the Word, and the Word was with God, and the Word was God. He was in the beginning with God. All things were made through him, and without him was not any thing made that was made. In him was life, and the life was the light of men. The light shines in the darkness, and the darkness has not overcome it. —John 1:1–5
The gospel of John opens with the breathtaking statement that the pre-incarnate Christ was the very Word of God, and therefore eternal deity, the second person of the Trinity, the agent of creation, self-existent—relying not on anything in creation for life but being the source of life both physically and spiritually—and the victor over the darkness of spiritual death. And Christ, as the Word of God, didn’t only make everything that was made, but it was he who spoke through the prophets of old:
. . . the prophets who prophesied about the grace that was to be yours searched and inquired carefully, inquiring what person or time the Spirit of Christ in them was indicating when he predicted the sufferings of Christ and the subsequent glories. —1 Peter 1:10–11
Which brings us right back to where our author began in the first verses of Hebrews:
Long ago, at many times and in many ways, God spoke to our fathers by the prophets, but in these last days he has spoken to us by his Son, whom he appointed the heir of all things, through whom also he created the world. —Hebrews 1:1–2
God’s word, as recorded in the Scriptures of the Old Testament and the New, speaks to us now— “Today” —with relevance, authority, and life-giving power and reveals him to us in all his radiant glory and holiness, his goodness, wisdom, and truth, and his merciful, gracious, faithful, pursuing, and redeeming love. Reading Scripture is how we hear God’s voice “Today.” And now, in the midst of this exhortation not to harden our hearts, but to strive to enter God’s rest, we are told that “the word of God is living and active, sharper than any two-edged sword, piercing to the division of soul and of spirit, of joints and of marrow, and discerning the thoughts and intentions of the heart. And no creature is hidden from his sight, but all are naked and exposed to the eyes of him to whom we must give account” (4:12–13).
What we are meant to learn here is not some sort of biblical anatomy lesson or the classification and relation of the invisible parts of our inward nature. The point being made is that when we read God’s living and active word which will accomplish the purpose for which he sends it, this word meant to make us wise and teach, reprove, correct, and train us in righteousness—this eternal word empowered by the Holy Spirit of Christ—God will use his word to delve deeper into our hearts than any man-made instrument could possibly go. Indeed, we cannot hide from this truth which discerns the thoughts and intentions of our hearts. “How often people think they are judging the Bible when just the opposite is true!”
In fact, while discussing this truth in our class the other day, Psalm 139 came to mind.
O Lord, you have searched me and known me!
You know when I sit down and when I rise up;
you discern my thoughts from afar.
You search out my path and my lying down
and are acquainted with all my ways.
Even before a word is on my tongue,
behold, O Lord, you know it altogether.
You hem me in, behind and before,
and lay your hand upon me.
Such knowledge is too wonderful for me;
it is high; I cannot attain it.
Where shall I go from your Spirit?
Or where shall I flee from your presence?
If I ascend to heaven, you are there!
If I make my bed in Sheol, you are there!
If I take the wings of the morning
and dwell in the uttermost parts of the sea,
even there your hand shall lead me,
and your right hand shall hold me.
If I say, “Surely the darkness shall cover me,
and the light about me be night,”
even the darkness is not dark to you;
the night is bright as the day,
for darkness is as light with you.
For you formed my inward parts;
you knitted me together in my mother’s womb.
I praise you, for I am fearfully and wonderfully made.
Wonderful are your works;
my soul knows it very well.
My frame was not hidden from you,
when I was being made in secret,
intricately woven in the depths of the earth.
Your eyes saw my unformed substance;
in your book were written, every one of them,
the days that were formed for me,
when as yet there was none of them. —Psalm 139:1–16
Our God knows us through and through. This knowledge is too wonderful for us and therefore moves those who have entrusted themselves fully to him to the only possible response: worship.
Hebrews was written to encourage Christians to hold fast to the truth in the face of pressure to return to the structures of Judaism in order to avoid persecution for their faith in Jesus Christ. The persecution would soon grow worse, not only from the Jews, but also from the Roman authorities. Christ’s defeated foe, Satan, was and still is seeking to destroy the image of Christ in the Church by any means he can. Christians have and will suffer. But these powerful truths of the omniscience and the life-giving power of God’s word are written to reassure us that there is no suffering so dark that God by his Spirit and his Word cannot penetrate. The devil may prowl around like a roaring lion, going to and fro on the earth seeking someone to devour (Job 1:7; 2:2; 1 Pet. 5:8), but he cannot see into the depths of human hearts, nor can he undo the work God has done there.
The author of Hebrews is also calling believers to examine their hearts to be sure they are truly in Christ, and aren’t simply trusting in the outward rituals and forms of religion to get them to heaven. By his Word, God will expose the places where we are hiding our trust in those things which cannot save. By his Word, God will point us to the only way of salvation, his Son, Jesus Christ, who alone is the Way, the Truth, and the Life (John 14:6). By his Word, God will pierce through stony hearts of unbelief and give instead hearts of flesh beating with new life. May the psalmist’s prayer be our own:
Search me, O God, and know my heart!
Try me and know my thoughts!
And see if there be any grievous way in me,
and lead me in the way everlasting! —Psalm 139:23–24
 Richard D. Phillips, Hebrews: Reformed Expository Commentary, (Phillipsburg, NJ: P&R, 2006), p. 138