If I had to single out one thing that played the greatest role in initially convincing me of the Bible's authenticity and the truth of Christianity, I'd choose the Old Testament prophecies, especially those concerning the Messiah. The specificity of some of the individual prophecies is powerfully probative, but the odds against so many of them being fulfilled in the person of Christ by coincidence are utterly breathtaking.
In about 700 B.C., the Prophet Isaiah specifically named the king (Cyrus) who would rebuild the Temple in Jerusalem, some 114 years before Babylonian King Nebuchadnezzar destroyed it and some 150 years before Persia conquered the Babylonians and its king (Cyrus) issued the decree to rebuild the Temple. Josh McDowell summarized it this way: "Thus Isaiah predicted that a man named Cyrus, who would not be born for about 100 years, would give the command to rebuild the temple which was still standing in Isaiah's day and would not be destroyed for more than 100 years."
Biblical scholar J. Barton Payne cited 574 Old Testament verses containing messianic prophecies, and countless others have listed and explained them, but my favorite compilation is by McDowell, who highlights some 60 of them as unmistakable predictions. Let me give you just a sampling with the humble suggestion that you read and contemplate these verses yourselves.
The Messiah would: reconcile men to God at painful cost to Himself; come from the seed of a woman (Genesis 3:15); be a Semite (Genesis 9:26); descend through Abraham (Genesis 22:18), Isaac (Genesis 21:12) and Jacob Numbers 24:17) and be from the tribe of Judah (Genesis 49:10); be a prophet, like Moses (Deuteronomy 18:15), a priest (Psalm 110:4), the judge (Isaiah 33:22) and king (Psalm 2:6); descend from Jesse's line (Isaiah 11:1) and David's line and be eternal king (2 Samuel 7:13); be God, the Father's Son (Psalm 2:7; 2 Samuel 7:14); ransom men and restore their righteousness (Job 17:3); exist before time began and be born in Bethlehem (Micah 5:2), and young children would be killed (Jeremiah 31:15); be given gifts (Psalm 72:10; Isaiah 60:6); be called Lord (Psalm 110:1); be "God with us" (Isaiah 7:14); be anointed by the Holy Spirit (Isaiah 11:2, 42:1); have zeal for His Father's house and reproach those who would violate it (Psalm 69:9); be announced in advance (Isaiah 40:3);