(Mat 25:46) “Then they will go away to eternal punishment, but the righteous to eternal life.”
As I’ve previously noted, “eternal” translates aionios, the adjective form of aion, meaning age and being the root of the English eon or aeon.
The question thus becomes whether aionios necessarily means “everlasting,” and it certainly can. But it can also mean “with everlasting effect.”
(Mark 3:29) “But whoever blasphemes against the Holy Spirit will never be forgiven; he is guilty of an eternal sin.”
Does “eternal” mean “sin that is everlasting” here? Consider the parallel in Luke —
(Luke 12:10) And everyone who speaks a word against the Son of Man will be forgiven, but anyone who blasphemes against the Holy Spirit will not be forgiven.
Luke defines “eternal sin” as “will not be forgiven,” not “will be tortured in perpetuity.” The sin is “eternal” because its effects will last forever.
Next consider —
(Heb 6:1-2) Therefore let us leave the elementary teachings about Christ and go on to maturity, not laying again the foundation of repentance from acts that lead to death, and of faith in God, 2 instruction about baptisms, the laying on of hands, the resurrection of the dead, and eternal judgment.
“Judgment” is over and over referred to in scriptures as an event. Now, it’s plainly an event with consequences that last forever, but the judging happens and is over.
Similar is —
(Heb 9:12) He did not enter by means of the blood of goats and calves; but he entered the Most Holy Place once for all by his own blood, having obtained eternal redemption.
To “redeem” is to pay a ransom. The saved had their ransom paid when Jesus died. The effect lasts forever. The event didn’t last nearly so long.
Therefore, “eternal” can refer to everlasting consequences of the noun modified. Clearly, “eternal punishment,” in parallel with “eternal sin,” “eternal judgment,” and “eternal redemption,” can refer to punishment with consequences that last forever, that is, punishment by death.
Fudge notes (The Fire that Consumes, p 122 ff) that Matthew 25:46 is parallel with Daniel 12:2 and John 5:29 —
Daniel: awake to everlasting life / awake to shame and everlasting contempt
John: rise to live / rise to be condemned
Matthew: go away to eternal life / go away to eternal punishment
The New Testament theology of the end times derives heavily from Daniel, and the parallels certainly suggest that the consequences of the punishment are everlasting, not the punishment itself.
Here’s another way of looking at it. What’s the antithesis of eternal life? Clearly, eternal death. Hence, “punishment” = “death,” which is, of course, a very common scriptural description of the fate of the damned.
Jesus said “eternal life,” not living eternally. Thus, he is speaking of eternal death, not dying eternally.
“Life” is a gift given at Judgment in the next age to the saved (John 17:2) . Death (or “punishment” ) is the penalty meted out at Judgment in the next age to the lost.
(John 17:2) For you granted him authority over all people that he might give eternal life to all those you have given him.
“Give” is aorist, meaning there’s a point in time at which God gives the gift of eternal life. The quality of the life is that it doesn’t end. You receive life aionios – for the age.
Just so, death or punishment is given at a single point in time. The quality of this death, unlike the first death, is that it doesn’t end. You receive death aionios – for the age.
Normal human life life is temporary. It’s finite. Like this age, it doesn’t last. But the second life is forever. It’s eternal life.
Normal human death is temporary. It’s finite. Like this age, it doesn’t last. But the second death is forever. It’s eternal punishment.
Does this cinch the case by itself? Certainly not. Rather, the weight of all the evidence must be considered. But Matthew 25:46 certainly doesn’t make the conditionalist position impossible.