Your asking, maybe somewhat poorly, one of the most contentious questions in Western History.

To address the poorly bit, the idea that Christianity was peaceful and never waged war in the name of religion is... Somewhat silly. When Charles the Hammer marched to war in 732, he did so under the banner of defending Christianity. The Christian kingdoms of northern Spain were waging war against the Muslims of Al-Andalus in the name of Christianity from the beginning. Constantine the Great, who converted the Roman Empire, marched under the banner of the Chi Rho.

Really the idea that Christianity was peaceful and never waged war in the name of religion just isn't true.

What they generally didn't do though, was wage war to conquer new lands in the name of Christendom. That is at the heart of your question really. And it's a huge question mark. Historians and scholars have all but taken long swords to each other's throats trying to answer it.

Why did the Latin West in the 11th century, suddenly decide to travel thousands of miles overland to invade the Holy Lands? Why did the Christians of Western Europe in the 11th century, suddenly have an interest in fighting a war for 'Christendom'? Why did God only then 'Will it'?

There's a bunch of answers and anyone who tells you they know it is probably stretching things because we've been debating the origins of the Crusades almost since they happened! Some answers are economic, that the Latin West had finally started to emerge from it's 'Dark Age' and began looking outward. Some are social, that the population had risen and this made Christians start looking for new lands. Some are political, noting the tense and bloody infighting of the Christian west at the time and how some of those men looked East to greener pastures. Other are of course religious, arguing a Christian awakening that had been building slowly for a long time.

And of course there are answers that combine multiples of these to try and figure it out.

There's a good book on the Crusades that I think can be dry, but it covers everything. The Crusades by Thomas Asbridge. The book kicks off with a great survey of the various answers to your question of why the Crusades happened and offers a great review of the first Three Crusades, which are generally the ones we all remember. It goes on, but I think the second half of the book is not quite as good as the first. Still, it'll give you a lot of things to think about