The best way to respond to the Jesus myth theory is to read the actual myths. They are counting on people not going to the primary sources. Once you read the myths, you will see that the parallels are only apparent when they rewrite them with biblical language.
Legend of the Death and Resurrection of Horus
The Attis of Caius Valerius Catullus
Mysteries of Mithra (This is not a primary source but is an introduction to Mithraism from both a non-Jesus myth and non-evangelical perspective)
