In this post I want to say something more about that. Specifically, how is salvation supposed to flow out of "divine union" with God? How's that supposed to work?
Again, the imagination required here is hard for us disenchanted, Western Christians. But here's a try.
In the early Christian imagination, and I'm speaking here of the church fathers, mortal human life was unstable and prone to dissolution, decay and death. This part, I'm guessing, is not too hard to understand. From dust we are and to dust we will return.
In the Incarnation corruptible human life was reconnected with God's own Life. Through the Incarnation an ontological merger occurred, a metaphysical connection was established between our life and God's Life. And through that connection our life is rendered immune to death, incorruptible.
Notice the difference here between life as consequence (getting to heaven) versus life as ontological change. It's a very different way of thinking about how salvation involves the defeat of death. Here's how St. Athanasius describes this in his famous treatise "On Incarnation":
For the nature of created things, having come into being from nothing, is unstable, and is weak and mortal when considered by itself...So seeing that all created nature according to its own definition is in a state of flux and dissolution, therefore to prevent this happening and the universe dissolving back into nothing, after making everything by his own eternal Word and bringing creation into existence, [God] did not abandon it to be carried away and suffer through its own nature, lest it run the risk of returning to nothing...lest it suffer what would happen...a relapse into non-existence, if it were not protected by the Word.
Note how the issues here are ontological. The created, mortal world is "unstable," in a state of "flux and dissolution" when separated from God. On its own, created reality is "dissolving back into nothing[ness]." That was the ontological predicament requiring rehabilitation, rescue, and saving, and it was accomplished through the Incarnation by (re)establishing an ontological connection between material reality and the Word. And it was precisely for this reason that the early church considered the Incarnation itself to be the critical, decisive act of salvation, and the resurrection the subsequent confirmation that death could not destroy mortal flesh when it had been mystically united with God's very Life.
All of this is pretty foreign to Western Christians. The metaphysical picture here is way too enchanted. As disenchanted Christians we can't fathom why the Incarnation made such an ontological difference, a difference vindicated in the resurrection of Jesus. Yet it was because of these enchanted ontological commitments that the early church focused salvation history upon Christmas and Easter. Disenchanted modern believers, by contrast, have lost this enchanted, ontological thread and have therefore focused salvation history on the only thing remaining in the story: Good Friday. And with that shift, with the marginalization of the Incarnation and Easter in salvation history, forensic notions of salvation focused on the death of Jesus came to predominate.
