Ellul is a universalist (a fact that he touches on in Subversion), and in the linked post he makes a well-reasoned case for his position. I'm not quite there myself; I see salvation for all as an outcome to be hoped for, but I'm not fully convinced of it. The bits and pieces that we find in the Bible regarding what happens after this life just don't give us a complete picture of what to expect.
The thrust of Ellul's logic goes like this:
If God is, he is all in all. There is no more place for nothingness. The word is an empty one. For Christians it is just as empty as what it is supposed to denote. Philosophers speak in vain about something that they can only imagine or use as a building block, but which has no reality of any kind.
The second and equally essential factor is that after Jesus Christ we know that God is love. This is the central revelation. How can we conceive of him who is love ceasing to love one of his creatures? How can we think that God can cease to love the creation that he has made in his own image? This would be a contradiction in terms. God cannot cease to be love.
If we combine the two theses we see at once that nothing can exist outside God’s love, for God is all in all. It is unthinkable that there should exist a place of suffering, of torment, of the domination of evil, of beings that merely hate since their only function is to torture. It is astounding that Christian theology should not have seen at a glance how impossible this idea is. Being love, God cannot send to hell the creation which he so loved that he gave his only Son for it. He cannot reject it because it is his creation. This would be to cut off himself.
Of course, Ellul's appeal here to human reasoning runs contrary to his arguments in other cases that human reasoning is insufficient to the task of understanding God, but in my experience virtually every theological position (including my own) runs aground on that contradiction at some point. Those that don't either reduce God to an overpowered human or spiral off into irrationality. To his credit, Ellul is careful in this essay to clarify that he does not assert his belief as dogma.
Ellul also argues against the idea that an individual's free will extends to the ability to reject salvation. I find his arguments on this point less convincing - which may be a sign of my own biases, since I still lean strongly toward the idea that love and free will are deeply intertwined.
Along the way he does, however, reinforce the point that rejection of the church is not necessarily rejection of God:
Without question we all know of innumerable cases in which people reject revelation. Swarms are doing so today. But have they any real knowledge of revelation? If I look at countless presentations of the Word of God by the churches, I can say that the churches have presented many ideas and commandments that have nothing whatever to do with God’s revelation. Rejecting these things, human commandments, is not the same as rejecting the truth.
I don't have the time or energy right now to examine the essay point by point; it's there for anyone interested in digging into it themselves. Regardless of whether eternal separation from God is a possible outcome for some, there is one thing I'm convinced of, and I suspect that Ellul would agree with me on this:
God did not intend for us to live our lives in a state of fear. Any church that uses fear of damnation to motivate its congregation - ever - has missed the point of the gospel.