"Take care, brothers, lest there be in any of you an evil, unbelieving heart, leading you to fall away from the living God."
The writer of Hebrews uses the description "the living God" to describe the Lord more than any other author in Scripture. In Hebrews 9:14, Jesus' sacrificial death, unlike the sacrifice of animals, can purify our conscience so we may serve the living God. Hebrews 12:22 calls Mount Zion (the heavenly Jerusalem) the city of the living God. While He is the Living God who we serve eternally in heaven, the writer of Hebrews also uses this title for God as a warning to us. Hebrews 3:12 warns of the possibility of falling away from the living God like the Israelites who hardened their hearts. Hebrews 10:31 goes on to tell us, "It is a fearful thing to fall into the hands of the living God."
It is fitting that Hebrews, a unique book in the Bible that sheds so much light on the Old Testament, would so prominently use "the living God" as a description of the LORD. The first use of this title is in Deuteronomy 5:26. In Deuteronomy 5, Moses is teaching the generation of Israelites who will enter the Promised Land God's instruction beginning with the Ten Commandments. In verse 21, he reminds them of the day the LORD spoke to all the nation on Mount Sinai (Exodus 19 -20). God spoke with a loud voice from the heart of the fire, surrounded by clouds and deep darkness. The people were afraid. The experience caused them to realize the holiness of God. They had heard the voice of the LORD and yet remained alive. But they did not want to risk death again. Instead of hearing directly from God, the people wanted Moses to be their intermediary. He would speak to the LORD and then relay God's instruction to the people. In verse 26 they say, "For who is there of all flesh, that has heard the voice of the living God speaking out of the midst of fire as we have, and has still lived?" (Hebrews 12:18-28 is a comparison of that day to the heavenly Jerusalem.)
This experience leads the people of Israel, who experienced all the miracles of deliverance from Egypt, to know and understand the LORD in a new and intimate way. They realized he was not a distant, impersonal God but One who would personally speak to them. His instruction was not just for the nation as a whole but each of them individually. They understood the fear of offending Him.
The LORD as introduced in Psalm 139 is living and active. He is impossible to escape. There is no place to hide from Him as even darkness is light to Him. He knows His creation so well; He knows what we will say before we speak. Of course, He knows and understands us intimately since He formed us. We exist because of Him, and He knows our beginning and end.
Just as Hebrews and Deuteronomy present the living God as One to be feared and loved, so does Psalm 139. Verses 19-22 describe those who harden their hearts as the wicked, who have malicious intent toward God as His enemies. Hebrews 3 is a warning not to have an evil, unbelieving heart like that first generation of Israelites who heard and saw the living God yet didn't enter the Promised Land because of their unbelief.
O LORD, such knowledge is too excellent for me. Your works are wonderful, my soul knows it very well. You have searched me and known me! Please continue to try me and know my thoughts! See if there is any grievous way in me and lead me in the way everlasting! Holy Spirit, please teach me to set my love, hope, and expectation on the Living God and no other. I want to be a citizen of the city of the Living God.