A great book related to the topic is Bat Yeor’s The Decline of Eastern Christianity Under Islam: From Jihad to Dhimmitude.

UPDATE#2: James Hakim serves as a PCA Pastor in Orange City, Iowa. Although, he is from Detroit, Michigan, his family is of Coptic origin, his father having been a Presbyterian elder in Egypt. He sent in this perspective as someone who holds North Africa dear to his heart:

I’m responding to your blog post/clip on the crusades; I would like to add a little context that is pretty undisputed, to supplement Dr. Vandoodewaard’s answer.

For centuries, North Africa was the most Christian region in the world. That region produced many great pastors and theologians, whom we now know as “church fathers”: Clement of Alexandria, Origen, Athanasius, Cyril of Alexandria, Tertullian, Cyprian, Augustine. 

The hard work and simple living of such simple societies produced a large amount of wealth and civilization that made them a tempting target for the Muslims. 

Just doing the math, one can see that the Muslim genocide of Arabia, Near Asia, and North Africa went basically unanswered for around 500 years. That in itself is astounding. 

What is more astounding is the amount of self-sacrifice that was involved for the people going on the crusades. Treasure could not be a motivation. One could barely carry enough back to compensate him for the cost of the journey. Many gave up the best years of their lives, leaving house and home behind. Many went, not knowing what would happen to their families while they were gone. 

But it was for their families, first, that they went–so that what had happened in North Africa would not soon repeat itself in Europe. And, too, being faced with the possibility of such horrors at home, it became no small thing that their own brothers and sisters in Christ had been facing the reality of these horrors (not merely the possibility!) for centuries.

And, of course, there is the cause of Christ, which is served more out of loving your brother than securing parcels of land in the Near East. It cannot be denied that this last was part of the “marketing” of the thing, and some good old North African “City of God” theology might have spared them this as well.

So, was Christianity responsible for the crusades? Christianity was responsible for sons and fathers and brothers and husbands willing to lay down their lives for the defense of those at home. Christianity was responsible for fair skinned Europeans being willing to die in the defense of men whose language and appearance was very different than their own–simply because they belonged to each other in Christ. Christianity was responsible for thousands of men doing thankless work, with no promise of any reward in this life. Christianity was responsible for many who loved not their life even to death.

The fall, and remaining sin, were responsible for a number of things that are now associated with the crusades. But there are many aspects of the crusades that pastors pray to see lived out in the boys and men of their congregations. And if anything will produce such character from the heart, it must be Christianity.

It is very easy to fingerpoint at Christians of another generation. If the crusading Christians could see how self-serving, worldly, inconsiderate, gender-confused, lazy, and demanding the Christians of today are, I certainly hope that they would not think that our “Christianity” is responsible for that! 

Yes, Christianity certainly teaches me to turn the other cheek when it is only my life or property that is at stake. But it also teaches me to love my brother and to love my neighbor, even unto the laying down of my life. And, sometimes, if it is the last option available to me in defense of brother and neighbor, loving my enemy will mean taking his life to prevent him from the bloodguilt of yet another murder at the judgment.

I know that you did not have so much time to say all of these things in the interview, and I am grateful for the answers that both you and Dr. Vandoodewaard have already provided. 

I just think that getting the actual dates of things in front of people, and having them swap shoes with believers from other centuries can be helpful. Perhaps then, they may see that Christianity really does result in much genuine good in the lives and history of Christians–and that the Crusades are actually an example of that!

As you made clear in your video answer, it is easy to demonstrate that Christians are still sinners. However, I think that we bring glory to Christ when we point out the good fruit that His grace has borne in the lives of many believers, even in this life.

Thanks again for taking the time to read this.

 UPDATE #3: Bill Vandoodewaard concludes with this caution.

By defending aspects of the Crusades as just war, I hope readers understand that we are not saying just war = preaching the gospel. Mixing the two in the wrong way has historically led to many difficulties and problems.  A helpful distinction is that soldiers who are Christians are called to be good soldiers, preachers are called to preach, not to careers of wielding the sword, and of course, Scripture does not call preachers to use the sword to encourage faith and repentance.