What is the Bible referring to when it mentions the first and second death?
The first death is temporal, and both the righteous and unrighteous people will suffer this one. This happens when a person dies and their spirit (breath of life) returns to God and the body is buried or disposed of:
“Then the dust will return to the earth as it was, and the spirit will return to God who gave it.” Ecclesiastes 12:7
However, a righteous person who might have died prior to the Second Coming of Christ will live once again at the resurrection of life, as the following verses describe:
“Do not marvel at this; for the hour is coming in which all who are in the graves will hear His voice and come forth—those who have done good, to the resurrection of life, and those who have done evil, to the resurrection of condemnation.” John 5:28–29
“For the Lord Himself will descend from heaven with a shout, with the voice of an archangel, and with the trumpet of God. And the dead in Christ will rise first. Then we who are alive and remain shall be caught up together with them in the clouds to meet the Lord in the air. And thus we shall always be with the Lord.” 1 Thessalonians 4:16–17
“Blessed and holy is he who has part in the first resurrection. Over such the second death has no power, but they shall be priests of God and of Christ, and shall reign with Him a thousand years.” Revelation 20:6
They might have experienced the first death, however, the second death has no power over them as these verses show:
“He who has an ear, let him hear what the Spirit says to the churches. He who overcomes shall not be hurt by the second death.” Revelation 2:11
“Blessed and holy is he who has part in the first resurrection. Over such the second death has no power, but they shall be priests of God and of Christ, and shall reign with Him a thousand years.” Revelation 20:6
The second death is an eternal death. This is what Christ refers to in the following passage:
“And do not fear those who kill the body but cannot kill the soul. But rather fear Him who is able to destroy both soul and body in hell.” Matthew 10:28
In essence, He is saying that there may be people who can kill your body temporarily, but you should only fear Him who is able to destroy you (your soul) eternally in hellfire.
There may be some people in this life who can bring harm and even death to your body, but they do not have power over your eternal fate. The first death is temporary, but the second death in the lake of fire is eternal.
“Then Death and Hades were cast into the lake of fire. This is the second death. And anyone not found written in the Book of Life was cast into the lake of fire.” Revelation 20:14–15
“But the cowardly, unbelieving, abominable, murderers, sexually immoral, sorcerers, idolaters, and all liars shall have their part in the lake which burns with fire and brimstone, which is the second death.” Revelation 21:8
Even the devil, himself, cannot escape the second death:
“The devil, who deceived them, was cast into the lake of fire and brimstone….” Revelation 20:10
So the soul is not immortal. It is not some invisible, intelligent substance that is separate from the body. And while men may hurt or even kill your body in this life, it is God who has full control over the eternal fate and destiny of men. Jesus sums it all up this way:
“And I say to you, My friends, do not be afraid of those who kill the body, and after that have no more that they can do. But I will show you whom you should fear: Fear Him who, after He has killed, has power to cast into hell; yes, I say to you, fear Him!” Luke 12:4–5