Weren't the prophecies just written after the events?

We have manuscripts containing biblical prophecies that were copied before the events Christians identify as their fulfillment.

The Great Isaiah Scroll

The Great Isaiah Scroll, dated by the Israel Museum to approximately 125 BC, contains all 66 chapters of Isaiah - including Isaiah 53. This physical manuscript existed before Jesus’ birth. Its description of the suffering servant could not have been invented afterward to match His crucifixion.

But he was wounded for our transgressions, he was bruised for our iniquities: the chastisement of our peace was upon him; and with his stripes we are healed.

The New Testament identifies this servant with Jesus (Acts 8:30-35). The manuscript establishes that the passage preceded Him; examining its fulfillment means comparing the prophecy with the testimony about His life and death.

Promises about the nations

The promises concerning Israel’s gathering are also demonstrably ancient. That same Isaiah scroll contains this promise:

And he shall set up an ensign for the nations, and shall assemble the outcasts of Israel, and gather together the dispersed of Judah from the four corners of the earth.

These words certainly were not written after the modern return of Jewish people to their homeland. Whether that return represents a partial or complete fulfillment requires examining the whole passage - related promises also include spiritual cleansing and renewal, so geographical return alone does not establish that everything has been fulfilled (Ezekiel 36:24-28).

Jesus explained the purpose of telling things beforehand:

Now I tell you before it come, that, when it is come to pass, ye may believe that I am he.