The Crusaders captured Palestine just after the turn of the first millennium. The Crusades were a series of religious wars between the 11th and the 15th centuries fought by the Roman Church. Many were expeditions by Christians attempting to capture the Holy Lands from the Muslims:
Pope Urban 2nd called the first Crusade in 1095. It started as a pilgrimage and ended as a military expedition. Urban was hoping to unite the Christian Church that had fractured into the East-West Schism.
The East-West Schism occurred in 1054 between the Eastern Orthodox and the Roman Catholic churches. There were many issues between the churches including whether leavened or unleavened bread should be used for the Eucharist, and who had overall jurisdiction the Bishop of Rome or the Archbishop of Constantinople.
The Byzantine Emperor Alexios I Komnos had asked for western volunteers to repel the Seljuk Turks from Anatolia. Knights, peasants and working classes from regions all over western Europe travelled to Constantinople then towards Jerusalem, hoping for rewards of many kinds: financial aid from the church, forgiveness from God for sins, feudal obligations, to gain glory and honour, or political and economic gain. They captured it in 1099 massacring both Muslims and Jews. They established the crusader states of the Kingdom of Jerusalem, the County of Tripoli, the Principality of Antioch, and the County of Edessa. Far from unifying the Roman Church, the First Crusade unleashed Catholic fury expressed in massacres of Jews and orthodox schismatic Eastern Christians. The People’s Crusade (the prelude to the first Crusade) of 1096 included the Rhineland massacre of thousands of Jews in France and Germany.
The First Crusade was followed by the Second to the Ninth. They were not always what the Church desired. Crusaders often pillaged and they often retained control of lands they captured rather than returning it to the Byzantines. The Fourth Crusade (1202-4) was requested by Pope Innocent 3rd to recapture Jerusalem from the Muslims via Egypt. It ended in the sacking of Constantinople (now called Istanbul) and the beginning of the end for the Byzantine Empire.
The population of Palestine was mostly Christian until 1187 when Saladin, a Sunni Muslim of Kurdish origin, led a military campaign against the Crusaders. He became Sultan of Egypt and Syria, and his conquests included Egypt, Syria, Upper Mesopotamia (north Iraq, northeastern Syria and southwestern Turkey), Hejaz, Yemen and into North Africa. Saladin took Palestine (and Jerusalem) from the Crusaders at the Battle of Hattin in 1187.
From the 1st to the 11th centuries the population of Palestine was mostly Christians.
From the 12th to the 14th century the majority shifted from Christians to Muslims.