It's a charge that comes and goes over the years. If you haven't heard it lately, you probably will soon. It goes something like this:
There have been stories about (semi-)divine men who came to earth, died, and were resurrected for thousands of years. The religions based on their worship even included baptism and a special meal. Early Christians simply borrowed elements from those religions to create stories about their own christ.And a lot of that is almost true. Almost. The collected wisdom of Christendom on the topic seems be:
1) The similarities between these ancient stories and Christianity are exaggerated and the differences are minimized.
For example, "resurrection" in these stories didn't really involve dead people getting up and walking around or it wasn't permanent. For instance, Osiris' dismembered corpse was reassembled, but he remained in the underworld. And their resurrection stories were tied to cyclical fertility/harvest rituals; they weren't once for all time events.
Likewise, "baptism" meant being bathed in blood from a freshly killed animal. Calling that baptism is stretching the term beyond recognition, and it's something first century Jews wouldn't have found at all interesting.
2) The real similarities are datable only to after Christianity appeared.
These "mystery" religions were still around at and after the time of Christ. If the similarities (e.g., communion) only appear in the historical record in the second or third century, which way did the borrowing go? The other way, obviously, as our traditions can be dated from the first century.
3) Post-exile Jews were the last people we'd expect to absorb or syncretize with another religion.
They were really, really opposed to allowing their religion to be corrupted by outside influences after that whole Babylon thing (c.f., Maccabean rebellion). That's not to say it could never happen, but it does mean the evidence has to be really solid for it to be believed.
So did early Christians borrow myths from other religions to create a Christ they could worship? The evidence does not support that hypothesis.
The Case for Christ
Jesus and Other Myths (Video)
The Resurrection: A Story No One Would Make Up
Pagan Virgin Births