Three ways I can think of for this to have been fulfilled even though we are now 1990+ years down the time line. 1. "Generation" can just as easily be translated "period" thus within this age/period of time this will be fulfilled. 2. The specific of the prophecy can be interrupted as having been fulfilled in AD70 and the actual Lord's latter return fulfillment of a greater prophecy in whole. 3. There are also the considerations of God's eternal perceptions of time (thousand years as a day) (time relativity) (the possibility that time is not sequentially serial but rather sequentially parallel). How deep do you want to go with it?
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