Why do all these facts sound so familiar? Mithras was worshiped by the Roman Constantine until 313 CE, when Constantine converted to Christianity after he superstitiously promised to convert if he won an important battle (In Hoc Signo Vinces ("in this sign you shall conquer") — IHS). When he became Emperor, Constantine made Christianity the official state religion of the Roman Empire.
Christianity changed in many ways to accommodate Emperor Constantine — for example, no one told Constantine that if he truly loved Jesus, he should "go, sell your possessions and give the money to the poor, and you will have treasure in heaven." No one told Constantine to obey Jesus and give up all his power and glory. No one told Constantine that Jesus was profoundly against fighting battles, killing, or playing power games. Constantine, not Christianity, cast the deciding vote at the vitally important first Council of Nicea in 324 CE.
None of these facts reflects badly on Christianity. Christianity had been an outlaw religion for almost 300 years; Christians were routinely martyred any time anyone wanted a scapegoat. But people should be aware that these ARE facts....