I want to begin this study with the “home team.” There is no such thing as an unbiased or completely objective observer, and therefore it is always wise to make certain one’s own house is in order before moving on to discuss someone else’s home. The crusades are the most prominent example of atrocities that can be truly charged to nominal Christianity.
Many Evangelicals will object immediately, arguing that the crusades occurred under auspices of the Catholic Church – but it is always difficult to make the distinction between Evangelicalism and Catholicism before the Reformation.
Most of the forerunners of the Reformation were simply one stream within the nominally Christian Western Church. Catholicism did not declare the doctrine of Salvation by Grace through Faith alone to be heretical until 1423, with the burning at the stake of John Hus; the first crusade began in 1096 and ended in 1099. Likewise, most crusades occurred before Thomas Acquinas combined Christian thought with Aristotelian metaphysics to form what is modern Catholic dogma.
Others might question the crusaders’ salvation, but this we are unable to judge (Matt. 7:1-5). Furthermore, the world will always view this as intellectually dishonest (whether it actually is, or not). While it may be true that the Crusades lack the evidence of a regenerated heart, for argumentation with those outside of the faith this distinction is a dead end, no matter how it contributes to the logical consistency of the Christian worldview.
The crusades were monstrous. The early crusades were advertised and sold as “just wars.” The initial motives of defending Constantinople and Christian pilgrims from alleged abuse, seems pure enough, but as soon as the nobles, generals and merchants took over these purposes soon took a backseat to slaughter and political backstabbing. Most likely, the first crusade ended with the slaughter and rape of civilians within Jerusalem. The second crusade halted on the way to Jerusalem to slaughter Jews living in Christian lands, and is considered by the Jewish people to be the first holocaust. The sacking of Constantinople during the fourth crusade was pure avarice at the instigation of bankers.
The crusaders were monsters, but were they self-consistent monsters? Bernard of Clairveux, who largely instigated the second crusade, along with other Christian leaders, decried the slaughtering of Jews as a violation of the crusader ideal.
If the New Testament is asserted as the central starting point for Christian thought, there is nothing in any letter or gospel to suggest that wholesale slaughter is permissible. The crusades also explicitly violated New Testament principles. For example, Christians are instructed that when the gospel meets hostility, to simply dust off their feet and move along; not to slaughter the hostiles (Mat 10:14-15). Judgment in such cases is clearly left to God. Likewise, Christian belief is that Salvation comes by Grace through Faith: to believe implies a matter of choice and will (John 3:15-20). The fact that this choice exists indicates that religious freedom is an underlying Christian principle.
My Presbyterian friends will disagree with my next point, but I am a Baptist and must answer from my own thinking. The Church, unlike Israel, is not a national government, and therefore does not possess the authority to make war.
Even if the crusades were initially justified, the crusaders became monsters. But, they were not self-consistent monsters, rather their atrocities are evidence of hypocrisy, since they are violations of their supposed Christian convictions.