"They will know you are mine if you love one another.", "...that they may be one as You and I are one." Looking at the 2,000 years of church history it could be argued that "Christians" have not loved one another or exhibited a "oneness" in purpose, doctrine or faith. The thirty years war which began during the reformation was not one war but many and waged all over Europe. It was especially brutal in the German states of the Holy Roman Empire. Charles the fifth, Holy Roman Emperor, turned Spanish armies loose on the German states and Bohemia leaving entire sections of the country empty of people, domesticated animals and tilled soil. In effect a great deal of the land was returned to wilderness. The German states suffered the worst but major wars were waged in Poland, France, Holland and as mentioned earlier Bohemia. This was not the end of religious wars in Europe nor the beginning but it was the worst.
Religion started it but in most cases religion was a pretext to gain territory, power and prestige or for some countries to make a bid for freedom in short they all had skin in the game wether they were ecclesiastical leaders or secular. The Roman church and their secular supporters had to much to lose to let the protestants slip from under their authority. In the beginning Luther did not want to cause a split in the church. He wanted the church to clean up its act and be what they claimed to be Christian. He surely knew better before he started but perhaps he hoped for divine intervention.
By in large I don't believe Christians were responsible for the torture, murder and brutal wars as much as it was those who claimed to be Christian. This was at a time when to be European was to be Christian. Virtually no one in Europe did not consider themselves to be Christians.
This problem began when Constantine legalized the faith and later it was made the official religion of the empire by Licinius. The many heresies of the early church along with the persecution helped keep the gospel message true. On the one hand the leaders had to become adept at countering heretics and the persecution pretty much guaranteed that nonbelievers would have little interest in becoming Christian.
This changed when the emperor favored the religion which made it attractive to those seeking political advancement. This was also when the church encountered a heresy that they couldn't attack directly because two emperors held the Arian view. It was also the belief of several of the barbarian tribes that overran the western empire. The church already had a problem with marginal believers seeking positions of leadership but the heresy hung on till the 9th century when it was finally put down by execution of its adherents and military action. The church leaders were determined to gain political power to enforce their dogma. This they did.
By end of the first millenium the Roman church was the only faith in western Europe except the pagan vikings and they were soon brought into the fold. Their authority was such that they could deny a king his crown. A king needed the church to legitamize his rule.
This influence came at a high price. The gospel is an affront to the natural man and can't really be grasped without a genuine born again experience. The church crafted a faith of merit and second chances that was more appealing to the semi-barbaric nobility of the time.
They also instituted more and more strict requirements for priest thereby cementing their control of the lesser clergy and intimidating the commoners. Access to holy writ was restricted by the requirement that the mass and the bible would be only in latin. They inserted priest between the believer and God with the confessional. It was about control and power.
Until the reformation the church did not have the gospel, the priesthood of the believer, the bible in the common language of the people or freedom of conscience and let us not forget, "justification by faith."
Lets move forward to the protestant fall from grace and the loss of the gospel. Education is one of the prime culprits in the death of so many protestant churches. The ordinary laymen is ignorant of what the bible really says and this gave overeducated unsaved clergy a free pass to reform the church again and make it more appealing to their, peers who were also overeducated, unsaved and found the true gospel an affront. The natural man will more likely believe a lie than the truth. When the early church began to turn control of the faith and the local church over to the "clergy" it started down a long hill to ruin. The body of believers, those that are saved and born again, are the best protectors of the faith. They are particular about what they believe and less likely to let anything new or novel sway their faith, they believe the bible more fervently, they love the Lord more intensely, and the true believer is the only one that really loves his brethren.
We are only sojourners in this world and our hope is in Christ Jesus, not politics or armies, or utopian quackery. Cling only to Jesus.