Simply answered, the Crusades were a defensive strike against the already growing threat of Muhammadan forces.
Many Christians did join the fight, primarily those under Catholic rule. By Christians, we are referring to a group of people defined as “Christian” by Catholicism. Many of these people were not “born-again” and were misled by the Catholic Church.
The reason many Christians signed up to fight was because the stories of what would happen to their families if the Muhammadan conquest reached their lands. Men would be killed, women and children would be raped and sent into slavery, both manual labor and sexual slavery.
Muhammadan forces were using tactics of the brutal Romans, giving cities they were about to invade three choices. Choice one: convert to Islam. Choice two: surrender and pay a degrading religious tax for keeping their old faiths. Choice three: die.
While the Crusades were used by the Catholic government as a way to spread their Christo-pagan religion, many soldiers were fighting for their families and way of life.
Today, Christians are still being killed for their faith in Islamic lands. Many Muslims, but not all Muslims, hold an unwarranted hatred for Christians. To this day, they still believe they require a retaliation strike for something that happened centuries ago and was started by their governments in the first place, who fully believed in the cause of their messenger Muhammad who claimed Allah gave him this cause.