Just wondering how can you say it won't last forever? Mankind will either spend eternity with or without Christ. They won't just cease to be, the bible says:
Matthew 25:41-46 Then shall he say also unto them on the left hand, Depart from me, ye cursed, into everlasting fire, prepared for the devil and his angels:For I was an hungred, and ye gave me no meat: I was thirsty, and ye gave me no drink.I was a stranger, and ye took me not in: naked, and ye clothed me not: sick, and in prison, and ye visited me not. Then shall they also answer him, saying, Lord, when saw we thee an hungred, or athirst, or a stranger, or naked, or sick, or in prison, and did not minister unto thee?Then shall he answer them, saying, Verily I say unto you, In as much as ye did it not to one of the least of these, ye did it not to me. And these shall go away into everlasting punishment: but the righteous into life eternal.
(it says everlasting)
Then Revelations also speaks on this.
Revelations 20:10-15 And the devil that deceived them was cast into the lake of fire and brimstone, where the beast and the false prophet are, and shall be tormented day and night for ever and ever. And I saw a great white throne, and him that sat on it, from whose face the earth and the heaven fled away; and there was found no place for them.And I saw the dead, small and great, stand before God; and the books were opened: and another book was opened, which is the book of life: and the dead were judged out of those things which were written in the books, according to their works.And the sea gave up the dead which were in it; and death and hell delivered up the dead which were in them: and they were judged every man according to their works.And death and hell were cast into the lake of fire. This is the second death. And whosoever was not found written in the book of life was cast into the lake of fire.
(The lake of fire is eternal it says, forever and ever. )
Either way man will spend eternity somewhere, why not choose life with Christ, it's the best decision anyone could ever make.