When we are praying about the result, say, of a battle or a medical consultation the thought will often cross our minds that (if only we knew it) the event is already decided one way or the other. I believe this to be no good reason for ceasing our prayers. The event certainly has been decided - in a sense it was decided 'before all worlds'. But one of the things taken into account in deciding it, and therefore one of the things that really cause it to happen, may be this very prayer that we are now offering. Thus, shocking as it may sound, I conclude that we can at noon become part causes of an event occurring at ten a.m. (Some scientists would find this easier than popular thought does.)
The imagination will, no doubt, try to play all sort of tricks on us at this point. It will ask, 'Then if I stop praying can God go back and alter what has already happened?' No. The event has already happened and one of its causes has been the fact that you are asking such questions instead of praying. It will ask, 'Then if I begin to pray can God go back and alter what has already happened?' No. The event has already happened and one of its causes is your present prayer. Thus something does really depend on my choice. My free act contributes to the cosmic shape. That contribution is made in eternity 'before all worlds'; but my consciousness of contributing reaches me at a particular point in the time series.
C.S. Lewis, Miracles, Appendix B (1947)