I can’t help but think that those who deny this aspect of the atonement are preventing their people from understanding the true nature of the love of God. After all, the very aspect of one dying for the other is surely a statement of a substitutionary death. All the words used are aimed at giving this impression.
“One died for all, therefore all died” This is not saying that therefore now all must attend to this death, but is saying that the “all” did indeed die* for the intended purpose. Also the fact that this was sin being condemned in the flesh means that this death took care of sin for us, in according to the purpose. There is great weight of clarity to be gained by giving consideration to what the purpose, the intention, was behind this action.
But the true message of the “substitution” atonement lies in that it is GOD who is suffering this absorbing of sin for us. That it is substitution because if he had not done it, it would not have been done, and the IT is the suffering that love undergoes for no other reason than that it IS love. If God does not forgive, we are dead. And we cannot earn his forgiveness, it is the gift of his love.
So God substituted HIS comfort if you like, by substituting what was going to be OUR discomfort for his own. The suffering that God and his son Jesus underwent would have been needless and unnecessary if the creation had been perfect, but because it wasn’t, it had to be compensated for by the action of the creator, and he suffered loss because of it.
Certainly this process may have involved the perfecting of Jesus as well, but that does not detract from the fact that it was all done for us. Yes people will not benefit if they do not believe, they will remain “dead”, but that does not take away the reality that he stepped in and acted on behalf of all mankind and suffered the same penalty (hell) which all who refuse release from death will experience.
If Jesus experienced what we who believe will then no longer experience because of his experience of it for us, I would call that substitution. It simply means that not everybody will take advantage of that substitution. The substitution was there, but they denied it.
*[this is forgiveness]