What is “the word of God” that Hebrews 4:12 says “is alive and exerts power”?

The context shows that the apostle Paul was referring to the message, or expression of God’s purpose, such as we find in the Bible.

Hebrews 4:12 is often cited in our publications to show that the Bible has power to change lives, and it is perfectly proper to make that application. However, it is helpful to view Hebrews 4:12 in its broad context. Paul was urging Hebrew Christians to cooperate with God’s purposes. Many of those were set out in the holy writings. Paul used as an example the Israelites who had been saved out of Egypt. They had the prospect of entering the promised land “flowing with milk and honey,” where they could enjoy true rest.​—Ex. 3:8; Deut. 12:9, 10.

That was God’s expressed purpose. Yet, the Israelites later hardened their hearts and did not exercise faith, so most of them failed to enter into that rest. (Num. 14:30; Josh. 14:6-10) However, Paul added that there was still “a promise of entering into [God’s] rest.” (Heb. 3:16-19; 4:1) That “promise” clearly is part of God’s expressed purpose. As did the Hebrew Christians, we can read of and cooperate with that purpose. To underscore that this promise is based on the Scriptures, Paul quoted parts of Genesis 2:2 and Psalm 95:11.

Certainly, it should touch us that “a promise of entering into [God’s] rest remains.” We trust that the Bible-based prospect of entering into God’s rest is really possible, and we have taken steps to enter. We have not done that by striving to keep the Mosaic Law or to earn Jehovah’s approval by other works. Rather, in faith we have happily cooperated with and continue to cooperate with God’s revealed purpose. In addition, as noted above, thousands of people around the globe have begun to study the Bible and to learn of the expressions of God’s purpose. Many of them are moved to change their lifestyle, to exercise faith, and to become baptized Christians. The way they are being affected is clear proof that “the word of God is alive and exerts power.” God’s expressed purpose, as outlined in the Bible, has already affected our life, and it will continue to exert power in our life.