Prophecy from the Almighty God's perspective, is speaking the words of the God to His people. Only certain humans in whom was the Spirit of Yehovah could prophecy, though there were many false prophets. They would speak God's words to them. As such they told the people how they have misbehaved and what they must do to recover their position. They also spoke of what God was going to do in the future. The word, prophet, is used to describe those writers in the Old Testament who wrote about what was to happen to the nation of Israel according as Yehovah spoke through the prophets. As such it is usually conditional, for long before Yehovah had told Israel what He required of them, He spoke punishment that would come upon them for their disobedience.
Under this heading however, I am going to consider only the prophecies concerning the coming of the Messiah. Some say there are about 700 such prophecies. Others say there are some 300-360 prophecies, while others talk about the 50 odd strong prophecies for which there is little argument that they apply to Jesus. For the purposes here, the number of prophecies above 50 matters little as we will soon see.
Mathematicians have sat down and worked out the probability that these 50 odd prophecies could come together in one person. It has variously been estimated as 1 in 10 to the 17 power. i.e. 1 in 100,000,000,000,000,000.
Some skeptics however say that the OT was rewritten by Christians after Christ died so as to fulfill these prophecies. They conveniently forget that we have historical evidence of the existence of the Septuagint, a Greek copy made of the Hebrew scriptures some 250 years before Christ. They also conveniently forget the secular manuscript evidence which independently testifies to the authenticity of the prophecies.
Josh McDowell in the first video below tells us that his book, Evidence that Demands a Verdict, was researched and written to refute the Christian beliefs. Listen to his reasoning as he fought to remain an atheist.