Theists: How can predetermined fate and free will coexist?

[henryp]

So; what is the difference if you go through door A and there is no envelope with the prediction?  What does it mean that you do not have freedom in this sense?
I could will to fly like a bird, but I'm not free to do so.

The envelope was an analogy. If what you will do is already known (doesn't matter by who or what) then you have no free will because your will is already determined. That simple. In this case the envelope knows your will, in your case the very God you believe in knows your will. 

Something cannot be known if it isn't determined, in this context.

The part I think you are missing, is that the person who is determining your will is you.  The filler of the envelop merely witnesses you further down the timeline making the choice.  Future you is still you, the order is just wonky because of agents who are able to view time in a non-linear manner.  But they are still viewing you choosing which door.  They aren't picking your door.  The envelop isn't picking the door.  Future you is picking the door.  And then when the time comes, you will become future you, and make the choice..

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It's a non-concept. You can't have outside of time or space. For something to even begin to exist it has to begin.

Are you sure?  I don't think God exists, but some wonky stuff happened back at 'the beginning' if the concept of 'beginning' is even something that applies ,which it may not.  Enough wonky stuff, that I think assumptions such as "everything has to begin" "and everything that exists is in our dimension" are shaky.