I apologise, but it's not exactly conducive to goodwill to call someone "in denial" and "very very biased" because they don't take the word of 'Archya S' off the internet at face value even though it goes against what academic historians say on the subject.
Which might well evidence that they are not exactly the same god seeing as the Roman Mithras was born from rock.
Also the winter solstice is not on Dec 25, and Dec 25th as a dating for Christmas comes significantly later than Paul. So it would mean Paul copied Mithraism, then people forgot about it, then later people also chose to copy Mithraism.
There is no real evidence of Roman Mithraism predating Christianity though. For example:
I propose to locate Mithraism's founding group among the dependants, military and civilian, of the dynasty of Commagene as it made the transition from client rulers to Roman aristocrats.3 The kingdom of Commagene on the Empire's eastern marches with Parthia and Armenia figures, more or less prominently, in all accounts of the transmission of Mithras worship, because the monuments and texts of Antiochus I, its mid-first-century B.C. ruler and the founder of a remarkable syncretistic Greco-Iranian royal cult, accord to Mithras a prominent place in the newly defined pantheon.37 It is, however, on the ending of the kingdom more than a century later that I wish to focus.38 The actual demise occurred in A.D. 72 with the deposition of the long-reigning Antiochus IV,
The Mysteries of Mithras: A New Account of Their Genesis Roger Beck
The Journal of Roman Studies, Vol. 88 (1998), pp. 115-128