And death and hell were cast into the lake of fire - Death and Hades (hell) are here personified, as they are in the previous verse. The declaration is equivalent to the statement in 1 Corinthians 15:26; “The last enemy that shall be destroyed is death.” See the notes on that passage. The idea is, that death, considered as the separation of soul and body, with all the attendant woes, will exist no more. The righteous will live forever, and the wicked will linger on in a state never to be terminated by death. The reign of Death and Hades, as such, would come to an end, and a new order of things would commence where this would be unknown. There might be what would be properly called death, but it would not be death in this form; the soul would live forever, but it would not be in that condition represented by the word ᾅδης hadēs - “hades.” There would be “death” still, but a “second death differs from the first, in the fact that it is not a separation of the soul and body, but a state of “continual agony” like what the first death inflicts - like that in intensity, but not in kind” (Prof. Stuart).
This is the second death - That is, this whole process here described - the condemnation, and the final death and ruin of those whose names are “not found written in the book of life” - properly constitutes the second death. This proves that when it is said that “death and hell were cast into the lake of fire,” it cannot be meant that all punishment will cease forever, and that all will be saved, for the writer goes on to describe what he calls “the second death” as still existing. See Revelation 20:15. John describes this as the second death, not because it in all respects resembles the first death, but because it has so many points of resemblance that it may be properly called “death.” Death, in any form, is the penalty of law; it is attended with pain; it cuts off from hope, from friends, from enjoyment; it subjects him who dies to a much-dreaded condition, and in all these respects it was proper to call the final condition of the wicked “death” - though it would still be true that the soul would live. There is no evidence that John meant to affirm that the second death would imply an extinction of “existence.” Death never does that; the word does not naturally and properly convey that idea.