Now traditionalists and conditionalists will want to know why such a spiritual interpretation of the threats of destruction is warranted, if the context of the judgment texts does not seem to suggest it. There are a number of ways to answer this question, but I will try to offer one particular line of reasoning.
Throughout Christian history, Jesus Christ has been taken as the interpretive key to Scripture -- not sociohistorical context, not rabbinical tradition, but Jesus Christ who is the fulfillment of all the scriptures. Thus Jesus is regularly portrayed in the church's iconography as holding the Bible in his hands; this is because he, as the Logos of God, guides its appropriate interpretation and no one else.
Now because Christ is the guiding principle of interpretation, he at times applies texts to himself which never would have been understood in this way in the original context. The most extreme example, perhaps, is Christ's words to his disciples at the last supper:
You will all become deserters; for it is written, ‘I will strike the shepherd, and the sheep will be scattered.’ But after I am raised up, I will go before you to Galilee (Mark 14.27-8).
This is a citation from the prophet Zechariah, whose words originally referred to something else altogether. In Zech 13 LXX, we find the following prophecy:
Awake, O sword, against my shepherds, and against the man who is my citizen, saith the Lord Almighty: smite the shepherds, and draw out the sheep: and I will bring mine hand upon the little ones. And it shall come to pass, that in all the land, saith the Lord, two parts thereof shall be cut off and perish; but the third shall be left therein. And I will bring the third part through the fire, and I will try them as silver is tried, and I will prove them as gold is proved: they shall call upon my name, and I will hear them, and say, This is my people: and they shall say, The Lord is my God. (Zech 13.7-9).
You do not get the impression at all, reading Zechariah, that he is speaking about the death of the messiah. There is no reason within this context to suppose that the striking of the shepherd(s) of God's people -- perhaps understood as the false prophets accused in vv. 1-6 -- is in itself salvific or remedial or redemptive, except insofar as they are removed from the populace. On the contrary, you only get the suggestion that it is an act of judgment akin to the judgment of Judah by the Babylonian captivity previous to this. The king and prophets and priests would be killed, many of the people would be killed, but a remnant would survive. This would be the way in which a fountain shall be opened for the house of David and the inhabitants of Jerusalem, to cleanse them from sin and impurity (v. 1).
Christ finds himself in this passage and interprets himself as the shepherd which is stricken. But there is nothing in the context of Zechariah's prophecy to suggest this. Yet Christ is the interpretive principle of the Scriptures; they speak about him and their meaning is informed by what happens to him and what he does.
The universalist engages in the same sort of thing when he interprets the threats of destruction of the wicked in spiritual as opposed to substantial terms. Christ died for all; his death is the propitiation of the sins of the entire world (1 John 2.2). His act of righteousness will make all human persons righteous (Rom 5.18-9). He will bring all people to himself (John 12.32) and will present all people in obedient submission and worship to the Father, so that God may be all in all (1 Cor 15.28). He is the Savior of the World (1 John 4.14), who brings salvation to all people (Tit 2.11). God's purpose is to unite everything under Christ (Eph 1.10). Jesus Christ is the savior of all people (1 Tim 4.10). Because Christ has died for all, all persons have therefore already died (2 Cor 5.14).
In light of all this, seeing what God has explicitly revealed about his purposes through Christ, the universalist is perfectly within her rights to interpret the destruction of sinners prophesied about in the Old Testament (and New) in spiritual terms -- even if the context does not ask for this nor permit it.
More should be said, and some day I will say it.