http://www.religioustolerance.org/chr_jcno.htm[/quote]
Interesting. Though he seems to be a bit biased and has misrepresented the correlation between Christianity and Mithraism by what I have read. A quick check shows that material evidence of Mithraism didn't show up in Rome until about the same time that the evidence of Jesus was showing up.
http://en.wikipedia.org/wiki/Mithraism#Mithraism_in_early_Rome
Christianity and Mithraism grew up together pretty much and eventually melded together when Mithraism disappeared which is why most christians celebrate Christmas on Dec 25th(The Festival of the Sun).
Christianity was actually more popular than Mithraism in general since Mithraism was exclusive in regards to membership. Since peasants, slaves, and women were all allowed to be Christians Christianity was much more popular. Among the aristocracy Mithraism was more popular.
It wasn't a Lion and Lamb but a Lion and Bull and as far as I have read (source Arkon Daraul - Secret Societies a history) these icons only developed in the later part of the Mithraist religion which actually started as something else entirely a couple thousand years prior to the first century AD. It was originally a cult based around the worship of a particular Djinn (not even named Mithras).