“And the wild beast was caught, and along with it the false prophet that performed in front of it the signs with which he misled those who received the mark of the wild beast and those who worship its image. While still alive, they both were hurled into the fiery lake that burns with sulfur.” ~Revelation 19:20

How do we know the lake of fire alluded to in Revelation is symbolic?
The wild beast and the false prophet are not literal.
The wild beast represents governments in opposition to God’s heavenly kingdom, and the false prophet represents the Anglo-American world power that encourages people to place their hope in democracy. (Rev. 13:11-13; 16:13)
After this, the Devil, Death, and the Grave are also hurled into the lake of fire.
Death and the Grave are not people or literal things, and the Devil is a spirit creature immune to the physical effects of fire. (Rev. 20:10,14)
This means that the lake of fire must be entirely symbolic, used to illustrate complete destruction.