What in the Hell is going on?
Universalism adds speculation to scripture to invent doctrine, just as Arminians and Calvinists do. They say after a person is thrown into the Lake of Fire, instead of being destroyed, they are -ah - purified and then they pop out of the Lake of Fire and joint the rest of the saints. How do you oppose speculation. They find something vague and pour their inventions into it.
I believe the body of Christ is divided by man-made doctrines. Only by sticking to what God's word actually says, and not bending scripture to fit doctrine by reworking the meaning of words such as "eternal" can we nourish unity.
My favorite response to those who say eternal does not mean eternal is John 3:16. If we shall not perish, then we have eternal life. If eternal did not mean forever and ever without end, then we would have to perish. So eternal means eternal.
So the bible says those whose name is not found in the Lamb's book of life are thrown into the lake of fire to suffer "eternal punishment." Whatever the punishment, it lasts forever and ever.
But to illustrate how speculation enters into doctrine, how many have been taught that the eternal punishment is eternal torment where the person being tormented has conscious awareness for eternity? Most I would guess.
But as the OP said, show me the verses that say that. What the bible actually says is Satan and his co-horts will suffer forever and ever -eternal torment. What the bible says about humans is (1) second death, (2) eternal punishment, (3) eternal darkness - i.e separated from God, (4) destroyed and (5) the smoke of the torment will rise forever and ever. So if we assumethat the smoke is emanating from on going torment, then we have a basis for the doctrine of "eternal torment." If we assume the smoke is a consequence of the torment in Gehenna, and simply illustrates the eternal consequence of that punishment, then we have no scriptural support for "eternal torment" but only the inventions of men.