What does it mean the gates of Hades will not prevail?

Jesus said “…I will build My church, and the gates of Hades shall not prevail against it.” (Matt. 16:18b).  This is said in response to Peters confession on whom Jesus actually is, that Jesus is the rock the church is built on and is the confession of all Christians; that Jesus is the Son of God - the eternal one.

  Hades is the unseen world, (Hebrew) Sheol or (Greek) Hades (cf. Luke 16:25). This is the place (state) of souls: Sheol is the place of departed spirits the only way to get there is to die. (Job. 38:17: the gates of death, Ps. 9:13 the gates of death.)  Those who died in the Old Testament times before the new covenant went to either Abraham's bosom (the faithful) or Gehenna (Mark 9:47 to be cast into hell fire—Gehenna.)

The gates are spoken of in the Old Testament-Jonah said, “The earth with its bars closed behind me forever.” He thought this is it, I died in disobedience and is in sheol.  Jonah’s prayer incorporates many phrases and figures of speech from the Psalms (18:6; 42:7; 31:22)

Job 17:16 “Will they go down to the gates of Sheol? Shall we have rest together in the dust?”

The earth with its bars. Job.38:17; Have the gates of death been revealed to you? Or have you seen the doors of the shadow of death?

Isa 38:10 I said, "In the prime of my life I shall go to the gates of Sheol; I am deprived of the remainder of my years."  Ps. 9:13 the gates of death.

Jesus said the gates of Hades Shall “Not prevail,” katischuo - to overpower: prevail (against).  

In its primary sense it means that though one may die they will be resurrected again - all believers have the hope of being raised in the likeness of Jesus’ resurrection. So death does not have a victory, the gates of Hades does not have a victory on those who believe. Jesus said in John 14:19 Because I live, you will live also.”

 John 11:25 Jesus said to her, "I am the resurrection and the life. He who believes in Me, though he may die, he shall live.”

John 5:25 "Most assuredly, I say to you, the hour is coming, and now is, when the dead will hear the voice of the Son of God; and those who hear will live.”

John 6:51 "I am the living bread which came down from heaven. If anyone eats of this bread, he will live forever; and the bread that I shall give is My flesh, which I shall give for the life of the world."

In a secondary sense the book of Acts says God added to the church daily despite the persecution. In the tribulation where the greatest number of Christians are martyred Jesus promises
Rev 12:11 "And they overcame him by the blood of the Lamb and by the word of their testimony, and they did not love their lives to the death."

There will always be the true Church because Jesus promised – I will build my church, and he will never leave or abandon it. There will always be a remnant of believers, large or small throughout history.

Rev. 1:18 "I am He who lives, and was dead, and behold, I am alive forevermore. Amen. And I have the keys of Hades and of Death."