Ah, a question dear to my heart – for it was part of leading me to the Catholic Church.
I faced this question in a much gentler situation. This happened loooong before I even thought about Catholicism. I had received one of those forwarded emails describing someone who needed prayer for something. Being in a charitable mood at the moment, I stopped and prayed for them. THEN, after the prayer, I read a little closer, looked at the dates, and realized that the original request was about 3 YEARS old. Because of the nature of the request, the situation would have been resolved one way or another long before now.
So, I suddenly wondered – had that prayer been wasted? Did it have any effect at all? After a few moments thought, I decided that since God knows all, the end from the beginning, and all time is “present” to Him, that yes my prayer now was as effective as it would have been 3 years earlier.
It helps to be a Dr. Who fan.
Remember, this was before I dreamed I’d be a Catholic someday.
But, having become settled on this long before, when I was introduced to the teaching that at Mass, we participate across time in the one sacrifice of the cross, and that the Eucharist we eat is the flesh of the one Passover Lamb that was slain once, yet is present in all time, this was easy for me to understand and accept.