The question of whether or not Paul made up Christianity by incorporating the mystery religions (i.e., the nature religions with the cyclical dying and rising gods) into Christian beliefs has become a popular subject in our comments section. After a quick search on the internet, I found an accessible article summing up the mainstream opinions of both Christian and secular scholars on this question. Here are a few excerpts from "Did Christianity Arise Out of the Mystery Religions?":
From a transcribed interview with Gary Habermas (an expert on resurrection scholarship) at the beginning of the article--the notion has been debunked:
Well, I think you're going to have to agree to this extent: virtually nobody is going to say that the Christians copied off these ideas [of the mystery religions]. That's a pretty radical idea. You could find it a couple of decades ago with some of the Bultmannian ideas. You can find a hundred years ago with the history of religions movement of similarities and comparisons. Both have died a death of a thousand qualifications.
Regarding secular scholars:
1. Even secular scholars have rejected this idea of Christianity borrowing from the ancient mysteries. The well-respected Sir Edward Evans-Pritchard writes in Theories of Primitive Religion that "The evidence for this theory...is negligible."
2. Samuel N. Kramer’s thorough work showed that the alleged resurrection of Tammuz (a fertility god of Mesopotamia) was based on "nothing but inference and surmise, guess and conjecture." Pierre Lambrechts maintains that in the case of the alleged resurrection of Adonis, no evidence exists, either in the early texts or the pictorial representations. The texts which refer to a resurrection are quite late, that is, from the second to the fourth centuries A.D. He reveals that for Attis there is no suggestion that he was a resurrected god until after 150 A.D. In the case of Adonis, there is a lapse of at least 700 years. If borrowing occurred, it seems clear which way it went (emphasis mine).
3. No less an authority than the great comparative religion scholar, Mircea Eliade, points out that not only is the idea of Christian borrowing from the Mysteries wrong but that any borrowing probably first began on the part of the mysteries: "Eliade said recent research did not support the theories that the origin of Christianity was influenced by pagan mystery cults. 'There is no reason to suppose that primitive Christianity was influenced by the Hellenistic mysteries,' said Eliade. In fact, the reverse may actually be true.... (See the online article for footnote information.)
There is more in the article, but to sum it up, and to add a few more points that I won't expand on now in the interest of space:
1. There are more differences than similarities.
2. The differences are significant and meaningful (e.g., the core messages are different).
3. The similarities in the mystery religions start appearing after the appearance of Christianity.
4. The pre-biblical, non-Pauline creed recorded in 1 Corinthians 15 stating that Jesus died and was resurrected is dated within two and seven years of the crucifixion (and of course the belief pre-dated even the creed)--too soon for such an outlandish myth to develop, and years before Paul even became a Christian.
5. There's still the matter of all the evidence for the resurrection that this theory cannot explain away. See Habermas's book, The Historical Jesus, for a summary of the pre- and non-biblical evidence supporting the physical resurrection of Jesus.