In this verse, Apostle Paul derived the image of God, as a "consuming fire" from Exodus 24:17; Genesis 4:24 and Isaiah 33:14. The Israelites saw the glory of God as a fire on top of Mount Sinai (Exodus 24:17). The glory of God is the sign of His presence and it is a fire. (See also Exodus 40:34-35). To witnesses of these events, Moses, Aaron, Abihu, and others (Exodus 24:9-10), it looked as if God really was “a consuming fire", a fire destroying everything. 

In Hebrews 12:29, Paul refers to the destruction in symbolic language of the sinful world order in the Day of Jehovah. Hebrews chapter 12 refers back to the inauguration of the Law Covenant at Sinai as an illustration of the inauguration of the New Covenant. With the start of Christ’s kingdom, the voice of God will shake the literal earth. In the typical event, God’s voice did shake the literal earth, but now God has promised more, saying: “Yet once more

[finally] I shake not the earth only, but also heaven.” The things shaken will be removed, because they are counterfeit [fabricated, false]; whereas, that which can not be shaken [only things true and just] will remain. Therefore, we, who we are to receive a kingdom unshakable, should serve God with grace, reverence and godly fear. Because [as written] our God is a consuming fire. 

During the Day of Jehovah, the same symbol of “fire” represents the process of purification of believers (1 Corinthians 3:12-15). This symbolic fire will destroy all “stubble” in order to purify all Christians’ characters. The Apostle Paul speaking only to those who had built their faith upon the sure foundation of Jesus Christ says: “Now if any man build upon this foundation gold, silver, precious stones [divine truths and corresponding character], wood, hay, stubble [errors resulting from tradition and the corresponding fickle character ]; Every man's work shall be made manifest: for the day shall declare it (2 Peter 1: 5-11); because it shall be revealed by fire, and the fire shall try every man's work of what sort it is.” 

On the basis of this Bible verse, surely even the most prejudiced would agree that a fire “burning” a spiritual work is not a literal fire. But fire is an appropriate symbol to illustrate the complete destruction of the state represented here by wood, grass and stubble. The fire will not be able to destroy the structures of faith and character built of gold, silver and precious stones of divine truth, having as a basis the rock of ransom sacrifice of Christ.