Mark Shea said "My Village Atheist was explaining to us rubes just the other day that the resemblance of Christ to various "pagan Christs" such as Osiris and Mithras is due entirely to the fact that Christianity borrowed motifs like death and resurrection and virgin birth from pagan mystery religions. It is not, of course, due to the fact that that the God behind the world was dimly perceived by the pagan mind which created portraits of dying and rising gods in reflection of that reality. Nope. There is no God who entered the world in the Incarnation. The apostles just got all het up about a rabbi they knew and somehow got the notion he rose from the dead in fulfillment, not only of his own words, not only of the Scriptures but of much in pagan imagination as well."
If the hypothesis that any element of a religion that also shows up at an earlier time in another religion invalidates the current belief then we should also be able to apply this hypothesis to science.
Thus penicillin and aspirin can't be true because ancient shamans used molds and tree barks to cure people.
The big bang theory can't be true because monotheistic religions said that the universe stated at a point in time.
The evolution theory can't be true because people like St. Augustine proposed a form of evolution.
The use of leaches by modern science to help people that have had a finger cut off can't be valid because people superstitiously used leaches to cure people in previous ages.
But if something can be true even though people previously thought the same or held some echo of the idea, then the hypothesis is invalid.
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Most of these "christ myths" are worthless anyway. People just take some random ancient deity like mithras and attach all of christ's qualities to him to make it look like he was there first. If anything, the mithrains got the (messiah) idea from jesus.