Our salvation rests in our being united to Christ as he died, and therefore raised with him into his life. If we are not united to Christ, then he did not bear our sins, and that punishment is yet to be payed. This is a uniting of each individual Christian to Christ. If this were a union of every individual human, then all of the sins of every individual human would have been born by Christ on the cross, and thus the punishment would have been payed already, and hell would be empty.
As a result of this, either our present actions, as coming to believe, have an effect on who was united to Christ as he died on the cross and as he rose, or Christ's bearing the sins of certain people effectually cause those certain people to eventually come to faith. For the purposes of this post, I won't argue for one or the other of these, although I believe that the second is the case. My point here is that both options entail the existence of two times, i.e., the coming to faith of a person and Christ's death, such that they are both "there" so to speak, such that when the Holy Spirit unites me to Christ, he is not uniting me to something that no longer exists for me to be united to, nor does he find that I do not yet exist to be united to Christ's death and resurrection. Thus Christ's death on the cross is as present for the Holy Spirit to unite me to as I am, and my sins are as present for God to lay on Christ, as Christ's death on the cross is for Christ to bear those iniquities on, and thus to pay for them on.
Open theism does not allow for this, since that part of the future which is dependent upon free willed actions does not exist until those actions are done, and so God does not know those parts of the future which are dependent upon the free will actions of others since they do not yet exist. If a person were united to Christ, then that would mean that the person's salvation was known by God while Christ hung upon the cross, but this cannot be if, while Christ hung upon the cross, our future acts of free will were unknown by God. Granted, many of those who believe in free will do not hold to the model of the atonement which this argument depends on, and my argument in that respect depends on the previous post.