The Quran and the Hadith command Muslims to fight non-Muslims and they are to be fought because of their beliefs:
Fight against those who (1) believe not in Allah, (2) nor in the Last Day, (3) nor forbid that which has been forbidden by Allah and His Messenger (4) and those who acknowledge not the religion of truth (i.e. Islam) among the people of the Scripture (Jews and Christians), until they pay the Jizyah with willing submission, and feel themselves subdued. Sura 9:29
And the Jews say: 'Uzair (Ezra) is the son of Allah, and the Christians say: Messiah is the son of Allah. That is a saying from their mouths. They imitate the saying of the disbelievers of old. Allah's Curse be on them, how they are deluded away from the truth! Sura 9:30
Sahih Muslim 33—It has been narrated on the authority of Abdullah b. Umar that the Messenger of Allah said: I have been commanded to fight against people till they testify that there is no god but Allah, that Muhammad is the messenger of Allah, and they establish prayer, and pay Zakat and if they do it, their blood and property are guaranteed protection on my behalf except when justified by law, and their affairs rest with Allah.Muhammad (SAW) is the Messenger of Allah, and those who are with him are severe against disbelievers, and merciful among themselves. Sura 48:29
O Prophet! Strive against the disbelievers and the hypocrites! Be harsh with them. Their ultimate abode is hell, a hapless journey's end. Sura 9:73
Verily, Allah has purchased of the believers their lives and their properties; for the price that theirs shall be the Paradise. They fight in Allah's Cause, so they kill (others) and are killed. Sura 9:111
Muslims are even forbidden from being friends with unbelievers:
O ye who believe! Take not the Jews and the Christians for friends. They are friends one to another. He among you who taketh them for friends is (one) of them. Lo! Allah guideth not wrongdoing folk. Sura 5:51
Fortunately many Muslims ignore such commands of their Holy book, but there is a majority that takes these commands seriously:
08/23/2011 Sudan (CDN)-More than seven months after Muslim extremists burned its church building, a Presbyterian Church of the Sudan (PCOS) congregation is still afraid to meet for worship, according to Christian sources.
The Rev. Maubark Hamad said his church in Wad Madani, 138 kilometers (85 miles) southeast of Khartoum, has not been able to rebuild since the Jan. 15 devastation due to the congregation’s meager resources.
“Nothing has been done for the burned church building; so far it has not been rebuilt,” he told Compass by phone.
Christian sources said they are increasingly fearful as Muslim extremists pose more threats against Christians in an attempt to rid what they call Dar al Islam, the “Land of Islam,” of Christianity.
“The increased challenges now faced by many Christians in North Sudan are something for which we need to pray very hard for the Lord to intervene,” said another church leader on condition of anonymity.
The PCOS building in Wad Madani was burned after a series of threats against its members by Muslims extremists, sources said.
“These anti-Christian activities
continue to be growing these days, aiming to cause fear among the believers in North Sudan,” said the church leader.
When PCOS leaders reported the case to police in Wad Madani, they were surprised to find officers reluctant to investigate. At press time the assailants had not been arrested.
Property damages to the church building were estimated at 2,000 Sudanese pounds (US$740); destroyed items included
Christian literature, Bibles in local languages, chairs, tables and a pulpit.
“Muslims target our church because they don’t want anything that is related to the church,” one church member said.
Christians in North Sudan are living beneath a blanket of fear since South Sudan seceded on July 9. Just one month after the South voted for independence from the predominantly Islamic North, pressures on churches and Christians have increased, with Muslim groups threatening to destroy churches, kill Christians and purge the country of Christianity.
One anti-Christian newspaper with strong ties to the North’s ruling party continuously advocates that North Sudan become a purely Islamic state and a purely Arab country. The Al Intibaha Arabic daily is well-known for provoking Muslims against Christians in Sudan.
North Sudan’s predominantly Arab population has intermingled with several indigenous peoples, leading some other Arab nations to regard it as not “pure Arab,” according to Operation World. Besides striving for an Arab-based ethnic-religious purging in North Sudan, Islamists may also be trying to counter estimated losses among adherents to Islam, with some estimating the Muslim population of the formerly unified Sudan recently dropping to about 55 percent from 61 percent.
Hostilities toward Christians by the Islamic government in Khartoum began to increase last year following a statement by President Omar al-Bashir, when he asserted that his second republic would be based on sharia (Islamic law) and Islamic culture, with Arabic as the official language(Source).