From Open Theism and Biblical Prophecy by Gospel Beyond Belief:
To understand premise 1, I want to explain what I mean by the strength of prophecy. I contend that the strength of Bible prophecy as a whole depends on the strength of individual prophecies and the quantity of such prophecies. Let us say for the sake of argument that Jesus was born in a manger on the evening of March 31, 4 B.C. in Bethlehem to parents of Davidic lineage. If this was so, then the following predictions, let’s say made in 500 B.C., would be on a scale of weaker strength prophecies to stronger strength prophecies:
A King will be born
A King will be born in Israel
A King will be born in Bethlehem
A King will be born in Bethlehem in 37-4 B.C.
A King will be born in Bethlehem in 4 B.C.
A King will be born in Bethlehem in 4 B.C. on March 31st
A King will be born in Bethlehem in 4 B.C. on the evening of March 31st
A King named Jesus will be born in Bethlehem in 4 B.C. on the evening of March 31st
A King named Jesus will be born in a manger in Bethlehem in 4 B.C. on the evening of March 31st
A King named Jesus will be born in a manger in Bethlehem in 4 B.C. on the evening of March 31st to parents named Joseph and Mary.
Prediction 10 is stronger than prediction 1 because you have to know much more about the future to be right about 10 than you do about 1. Specificity is not the only measure of the strength of prophecy, however. The amount of such prophecies also figures in the calculus. It could be the case that one prediction of value 4 is stronger that two predictions of value 3, but that would depend on the scale. But the obvious point remains that the more correct predictions the Bible makes the stronger Bible prophecy will be. But keep in mind that prophecies of low strength are still true.
I contend that the individual Old Testament prophecies concerning Jesus’ first advent fall in the 2-3 range in the (imperfect) scale of my example, but they do not reach to strength 4. This leads me to rate the overall strength of Bible prophecy, taking into account quantity as well as quality, in the 2-3 or 3-ish range. This is fairly low.
Open theism provides a better model for why Bible prophecy would have strength 2 or 3 and not 9 or 10. If the future is open then there is a lot of historical wiggle room that God gives freedom. Given this freedom, and given God’s unthwartable sovereign plans, then we wouldn’t expect Bible prophecy to be much higher than 2 or 3. Open theism is often compared to a chess game in which a grandmaster will always beat a novice even though the grandmaster does not know in advance what moves the novice will make. The grandmaster’s plan of victory is assured. God’s plans are assured even though the individual moves might not be known in advance. The grandmaster will win, even though we don’t know that it is by capturing the rook and forcing checkmate on move 14, say. In other words, the reason the strength of Bible prophecy is low is that Open theism is true.