This should be the most common response to Creationists who insist that the theory of evolution is “just a theory”. And that feeling of being taken aback when you say “well, Jesus is just a theory” is the same feeling that normal people get when Creationists say that “evolution is just a theory”.
Make no mistake. Jesus is a hypothetical construct posited to make sense of Christianity, much in the same way the theory of evolution is posited to make sense of biology. The evidence for evolution is much better than the evidence for the existence of Jesus. So if Creationists want to doubt the theory of evolution because it’s “just a theory”, then they should be even more doubtful of the existence of Jesus based on the same logic. This is based on the simple fact that the hard sciences have more evidence to work with than history.
On the flip side of that, if the existence of Jesus is beyond reasonable doubt for the Creationist, then how much more solid is the existence of evolution? They are really stuck between a rock and a hard place if they assert that one is open to questioning and the other one is beyond questioning. This statement should really expose the hypocrisy of their “it’s just a theory” statement.