What happened to these Christians in the past?

In the late 19th and early 20th centuries, Palestine was part of the Ottoman Empire.  At the end of the first World War, the League of Nations established “mandates,” or temporary oversight bodies, in various parts of the former empire. These were to provide administrative assistance until the areas could become self-governing, and in all the areas except Palestine the mandates became independent countries. Palestine was under the British Mandate, and in 1917, Lord Balfour of England issued a letter expressing support for the establishment of a Jewish homeland in that area. His letter, known as the “Balfour Declaration,” specified that no harm should come to people of other ethnic groups living in the region. After 1917, Jewish immigration to Palestine began to increase and accelerated in the 1930’s as persecution of Jews in Europe intensified.

By 1947, the Jewish people comprised 33% of the population but still owned a very small percentage of the land. When Israel was established in 1948 by the United Nations, the new entity was given 56% of the land west of the Jordan River that was considered the British Mandate of Palestine. Palestinians, who comprised a majority of the population and were not consulted, were left with a much smaller portion, and resisted with the support of neighboring Arab countries. The war that followed resulted in the addition of more land to Israel, which now includes 78% of the total. Palestinians were left with 22%, divided into two disconnected territories of the West Bank and Gaza Strip. In 1967, following the Six Day War, Israel seized control of both these areas, the Sinai Peninsula (which it returned to Egypt) and the Golan Heights, which it captured from Syria.

Map from the US State Department:
https://2001-2009.state.gov/r/pa/ei/pix/b/nea/20958.htm  

Most major Palestinian towns had significant Christian populations before Israel’s establishment in 1948, although they were often minorities. Shortly after its establishment in 1948, Israel destroyed several Palestinian villages with large or majority Christian populations, such as Iqrit and Kafr Bir’im. They were among around 500 Palestinian towns emptied of their native inhabitants, with many destroyed. Over 750,000 Christians and Muslims were either forced out or fled in the wake of massacres committed by Zionist troops with the aim of causing Palestinians to leave. Many of those fleeing went to neighboring countries, thinking they were only going for a short time, but Israel passed laws saying they could not return. Today only people with Jewish ancestry can move to Israel. There are 9.17 million displaced Palestinians worldwide (as of 2021), according to the BADIL Resource Center for Palestinian Residency and Refugee Rights. About 5.7 million live in impoverished refugee camps in Gaza, Lebanon, Jordan, Syria and the West Bank, waiting to return to the land of their ancestors.

The Melkite Greek Catholic Church ruins in Iqrit https://commons.wikimedia.org/w/index.php?curid=10794268

In his book Blood Brothers, Father Elias Chacour describes being expelled with his family from his hometown of Bir’im and later watching from a nearby hilltop as his village was destroyed. Of life in the village, he writes, "We were all proud of our school, just as we were proud of the lofty church across the small courtyard, which the people of Biram had also built stone by stone.” The church was the only building left standing when the Israeli air force bombed the town and destroyed all the homes in 1953, just after the Israeli Supreme Court had ruled that the villagers had a right to return.

Another mostly Christian village forced to evacuate by Israelis in 1948 was Maaloul, near Nazareth. The village had two churches, one Catholic and one Greek Orthodox, as well as a small mosque and cemeteries for Christians and Muslims. After being forced out, many villagers went to Lebanon as refugees, with others remaining in the Nazareth area but forbidden from returning to their homes, which were eventually bulldozed. Israel allows villagers to return to Maaloul once a year, on Israeli Independence Day, when they spend time among the ruins of their churches and mosque, while Israelis celebrate.