Just like St. Augustine f.e., I believe that God lives outside of time. Unlike him, I have come to know that time is but another dimension in our universe, the fourth dimension of space-time. So time, as shown in relativity theory by Einstein, can f.e. be bent just like any other of the four dimensions.
But if God lives outside of time, he can observe time just as we can observe space. When we look at things, we can pretty much decide upon the perspective we take and view the whole stretch of, let's say, the length of what we are looking at, if the object is small enough to not escape our view port.
Now, the Bible tells us that God holds the universe between his thumb and his forefinger. That would by no means escape his viewport.
Let us make an example. I can touch the middle of the desk in front of me, and then touch it's left end, before touching its right. If time is only a dimension, and I am not a slave of time, that means subjected to it as we are (we experience time as a half dimension, since we can only travel through time forward, and are not free to select the speed in which we travel), God can touch time in whatever order he wants.
In that scenario, Christ can easily be slain before the world even exists, touching a later point in time (after creating it in the first place, since, as a dimension of this universe, it did not exist before creation). One could view it just like the creation in Gen 2: God made man, and then put him into the garden. He made man before anything grew (maybe even before plants even were created), but put him into creation in the afternoon of the sixth day (i.e. a much later time).
Thinking this through, the experience of the cross becomes something outside of time. The moment I accept the work of Christ at the cross, the cross happens for me. This event outside of time becomes real and manifests in time at a set time.
God knows the end from the beginning. (Isa 46:10)
God planned it all before time was. (Eph 1:4)
This is how God works. We pray, and he puts the answer into our future. We prophecy, as he reveals to us what he put into the future already. He said: it is finished, as he had accomplished all, including future things.