Answering the question of what the second death is could be done briefly and concisely, because the Bible itself gives a clear description of what the second death is. We find this in the book of Revelation, which is also the only book of the Bible in which the term occurs.
Revelation 20
14 And death and Hades were cast into the lake of fire. This is the second death: the lake of fire.
15 And if anyone was not found written in the book of life, he was cast into the lake of fire.
The definition of the second death is given by Scripture itself. This is the second death: the lake of fire.
the thousand years
The beginning of Revelation 20 describes that at the end of this eon a new age will begin, in which Satan will be bound for a thousand years, so that he will not lead the nations astray (Rev.20:1-3). Christ will reign as King in this eon, and that is why this period is also called: the Messianic Kingdom of Peace.
resurrection from the dead
Before the thousand years, many of the dead will be raised to enter the Kingdom. This is called the first resurrection (:5-6). The first resurrection is not one moment in which the dead are raised, but is a series of resurrections in which believers are raised.
The first resurrection is a kind of resurrection, namely a resurrection from the dead. It is called resurrection from the dead, because some rise, but the rest remain in death. The first resurrection is a resurrection from among the rest of the dead.
resurrection of the dead
The part that remains behind in death will only be resurrected after the thousand years to appear before the great white throne (:11). This is the resurrection of the (remaining) dead (:5). They died as unbelievers and are judged before the great white throne (:12).
Ecclesiastes 12
14 For God will bring every deed into judgment for every secret thing, whether good or evil.
rectified
Before the great white throne all, and everything, is judged, that is set right. God’s judgment is true (Rom.2:2). There the truth will come to light (Rom.2:6-9) and nothing will be hidden. The evil that someone has done will be repaid (Rom.2:8) and for many this judgment will be an experience of tribulation and distress (Rom.2:9).
God’s goodness
But it is God’s goodness that He judges, He sets right all that is crooked. After the judgment at the great white throne, there will no longer be such a thing as injustice, but neither will it continue. Victims receive reparation and perpetrators are recompensed for their evil.
Paul rightly calls God’s judgment: gospel, that is: good news.
Romans 2
16 in the day when God shall judge the secrets of men, according to my gospel, by Jesus Christ.
raised in transience
Those who are judged before the great white throne are not raised in incorruption. Their old life is judged and they are directed into the old body. That is why they die a second time, just like, for example, Lazarus, the daughter of Jairus and the young man of Nain.
They die a second time by being thrown into the lake of fire, which is the second death (Rev.20:14). They receive imperishable life when death is abolished. More about that in a next blog.