Atheist: In order for me to consider a prophecy to be truly fulfilled, the prophecy must not be able to be fulfilled by people who already believe in it.
Answer: This is a convenient way to dismiss anything. It's kind of like the new atheist requirement of acceptable eye witness accounts of Jesus--that is, they would accept an eye witness claim only if the eye witness wasn't a Christian. In other words, they will only accept testimony from someone who didn't believe what they saw.
As vague as you might think some of these prophecies are, you cannot simply plug in any arbitrary historical figure that would fit the way Jesus does.
Ask any atheist friend who this is and where this quote is from:
"Surely he took up our pain and bore our suffering, yet we considered him punished by God, stricken by him, and afflicted. But he was pierced for our transgressions, he was crushed for our iniquities; the punishment that brought us peace was on him, and by his wounds we are healed."
They will all say "Jesus", and they will all say it is from the New Testament. They will be right about Jesus, but they will be wrong about where it is from. It is from the book of Isaiah, written about 700 years before Christ. I won't provide a link because you will say it is biased. So I challenge you, go do your homework and come to your own unbiased conclusion as to when the book of Isaiah was written.