We know some prophecies in the Bible (e.g., the prophecy in Daniel 8 about the Greeks defeating and replacing the Medo-Persian empire) are true because they have been fulfilled (the Greeks did defeat and replace the Medo-Persian empire1) — i.e., what they said would happen has happened. Because these prophecies have been fulfilled, it gives us confidence that the other, as yet unfulfilled, prophecies will one day be fulfilled. Through the fulfilled prophecies, God has shown that he is true to his word and that, therefore, we can trust (have faith in) what he says about things that are yet to happen.
1. ‘Alexander the Great (356-323 BC)‘ from BBC History; ‘Ancient Greeks: Greeks at War‘ from BBC Primary History (both accessed 03/03/11).