Well, the word extreme means 'furthest from the norm''. So, an extremist Muslim is the one furthest from the norm. What is the norm? The norm (as per the billion Muslims in South East Asia) is to be a law abiding citizen -- nonviolent, peaceable, committed, spiritual. The extreme is to be drastically different than that norm -- violent, lawless, crass.
For Christians, I suppose the norm is to be peaceable (if a little rude and condescending at times), committed, and spiritual, and the extreme is to be violent, warmongering and aggressive.
If you define a ''real Christian'' as a nonviolent spiritual person committed to Jesus (who was also a nonviolent spiritual person), then I see no reason that the ''normal'' majority Muslim can't define a ''real Muslim'' as a similarly peaceable person, thus for you to say ''those who commit atrocities in the name of God are not real Christians'' is just as valid as when your common Muslim says 'those who commit atrocities in the name of Allah are not real Muslims''.