Answer:
To obey the Pope's call to free the Holy city from the infidels and ensure access for pilgrims. St Bernaud of Clairvaux wrote in 1140, Of mighty soldier, oh man of war, you now have something to fight for. If you win it will be glorious. If you die fighting for Jerusalem, you will win a place in heaven.
To be forgiven for past sins. The Pope offered forgiveness for anyone who took part. This was important for knights who had killed many people in battle.
To see the world, have an adventure and prove their bravery.
To get land overseas. This was tempting for a younger son who would not inherit his father's lands.
Serfs, peasants who belonged to their lord, joined the Crusades because the Pope promised them their freedom if they went.
To gain wealth.
Kings encouraged troublesome knights to go on Crusade because it got them out of the country.
In the 1000s, tension increased between Muslims and Christians
The Taking of Jerusalem by the Crusaders
Some historians would argue that whilst the primary motive may have been religious, many Crusaders got side-tracked by their greed and lust for power. This may explain why they slaughtered fellow Christians in Constantinople during the first Crusade and took control of Edessa, which was not on the route to Jerusalem and did not contain any holy sites.
