By the third century, Mithraism was officially sanctioned by the Roman Emperors and according to the fourth century Historia Augusta, the emperor Commodus participated in its mysteries.
Women however were excluded from Mithraism and some scholars argue that the eventual rise in popularity of Christianity was because it offered women a role, albeit usually a subordinate one.
As Tylluan pointed out the early Church had quite a few problems with Mithras. Roman Mithras was perhaps the greatest rival to early Christianity for many reasons. As well as being a popular Pagan religion practiced by the Roman army, Mithraism had many similarities to Christianity.
As Christianity gained strength and became the formal religion of the Roman Empire, the 'Cult of Mithras' was one of the first Pagan cults to come under attack in the fifth century. Temples of Mithras, like most other Pagan temples, were destroyed and Churches built on them. It is quite fortunate that some remains e.g. the temples at Hadrian's Wall, survive.
Mithraism was the last state Pagan religion in Europe however Mithras still influences our lives today even if we do not realize it. The Christian Church could not eradicate Mithraism completely and in particular could not prevent people celebrating his birthday. The Church therefore took over the festival and we know it as Christmas - December 25th!!
Here is a picture of the remains of the Temple of Mithras in London:
Shipscook
ever since direct flights to Manchester I've been able to avoid London,It's so expensive now too. 
This is somewhere very close to my office that I must pop down to and have a look, funny isn't it how you can scramble all over places like Rome, Luxor or Ephesus and not take advantage of things on your back door.