Of course atrocities have been committed in the name of Christianity in the past. I think we just differ on some particulars-how long ago, how bad, and whether or not those perpetrating the violent, cruel acts were really motivated by the teachings of Jesus Christ or by a lust for power where religion was invoked to act as superficial justification. For example, the intertwined power structure of the Church and Royal Families made any slight or disobedience to the Church a perceived threat to the status quo-which resulted in the barbarism of the Inquisition.
I think my main two points are this:
If one examines and takes to heart the teachings of Jesus Christ, the main prophet/god of the Christian faith, he or she would be very peaceful and compassionate towards all life.
If one examines and takes to heart the teachings and life of Mohamed, the main prophet of the Muslim religion, he or she would be inclined ,to and justified in, committing all forms of barbaric violence and brutality towards fellow human beings.
Those last two points are, in my eyes, indisputable.