But here is a recent fatwa (legal ruling) which explains nicely the prohibition of the building of new churches in Islamdom.
Official Saudi Fatwa of July 2000 Forbids Construction of Churches in Muslim Countries
The website www.kalemat.org posted a fatwa issued on July 3, 2000 by The Permanent Council for Scholarly Research and Religious Legal Judgment, an organ of the Saudi Ministry of Religious Endowments, forbidding the construction of non-Muslim houses of worship in Muslim countries. The fatwa stated that it is forbidden to allow non-Muslims to establish a foothold in the Arabian Peninsula, to receive Saudi citizenship, or to buy property there. In addition the Kuwaiti daily Al-Siyassa reported that Kuwaiti MP Walid Al-Tabatabai announced in statements earlier this winter that he was opposed to the establishment of houses of worship for non-Muslims in Muslim countries.
The following are excerpts from both sources:
All Religions Other Than Islam are Heresy
The Saudi fatwa reads as follows: "The Permanent Council for Scholarly Research and Religious Legal Judgment has studied the queries some individuals brought before the Chief Mufti… concerning the topic of the construction of houses of worship for unbelievers in the Arabian Peninsula, such as the construction of churches for Christians and houses of worship for Jews and for other unbelievers and [the question of] the owners of companies or organizations allotting a fixed place for their unbelieving workers to perform the rites of unbelief.
"After considering the queries the Council answered as follows:
"All religions other than Islam are heresy and error. Any place designated for worship other than [that of] Islam is a place of heresy and error, for it is forbidden to worship Allah in any way other than the way that Allah has prescribed in Islam. The law of Islam (shari'a) is the final and definitive religious law. It applies to all men and jinns and abrogates all that came before it. This is a matter about which there is consensus.
"Those who claim that there is truth in what the Jews say, or in what the Christians say - whether he is one of them or not - is denying the Koran and the Prophet Muhammad's sunna and the consensus of the Muslim nation… Allah said: 'The only reason I sent you was to bring good tidings and warnings to all [Koran 34:28]'; 'Oh people, I am Allah's Messenger to you all [Koran 7:158]'; 'Allah's religion is Islam [3:19]'; 'Whoever seeks a religion other than Islam, it shall not be accepted from him [3:85]'; 'The unbelievers from among the people of the Book [i.e. Jews and Christians] and the polytheists are in hellfire and will be [there] forever. They are the worst of all creation… [98:6].'
"Therefore, religion necessitates the prohibition of unbelief, and this requires the prohibition of worshiping Allah in any way other than that of the Islamic shari'a. Included in this is the prohibition against building houses of worship according to the abrogated religious laws, Jewish or Christian or anything else, since these houses of worship - whether they be churches or other houses of worship - are considered heretical houses of worship, because the worship that is practiced in them is in violation of the Islamic shari'a, which abrogates all religious law that came before it. Allah says about the unbelievers and their deeds: 'I will turn to every deed they have done and I will make them into dust in the wind [Koran 25:23].'
"Thus the 'ulama agreed that it is forbidden to build heretical houses of worship - such as Christian churches - in a Muslim country, and that it is forbidden for there to be two directions of prayer coexisting in a Muslim country, and that there should be no symbol of unbelief, neither churches nor anything else. They agreed that it is obligatory to destroy any church or other heretical house of worship that was built after [the advent of] Islam, and it is forbidden to oppose the ruler in the matter of its destruction, and he must be obeyed.
"The 'ulama agreed that building heretical houses of worship, such as churches, in the Arabian Peninsula is the most weighty of sins and the worst of crimes, because there are reliable and explicit sayings of the Prophet [hadith] that prohibit the existence of two religions in the Arabian peninsula [i.e. another religion in addition to Islam], among them the Prophet's words that were related by [Imam] Malik and others and were recorded in the Sahihayn [the two most authoritative collections of hadith for Sunni Muslims compiled by Al-Bukhari and by Muslim]: 'There shall not be two religions together in the Arabian Peninsula.'
"The Arabian Peninsula is Islam's sanctuary and its basis. It is forbidden to allow or permit unbelievers to penetrate it or to receive citizenship there or to buy property, not to speak of building churches for the worshipers of the cross. There is no place in the Arabian Peninsula for two religions, but only for one - the religion of Islam, sent by Allah through Muhammad, His Prophet and Messenger. There will not be two directions of worship there, but just one single direction - the direction of the Muslims, towards the Ka'ba in Mecca. Praise Allah who enabled the rulers of these lands to ward off these heretical houses of worship from the pure Islamic land.
"[We turn to] Allah, to whom we complain about the heretical houses of worship that the enemies of Islam brought, like the churches and others, to many Muslim countries. We ask Him to protect Islam from their cunning and deceit.
"If one allows or consents to the establishment of heretical houses of worship, like churches, or if one allots a fixed place in a Muslim country [for them to worship] - this is the worst sort of aid to unbelief and of bringing their rites into the open, [in defiance of what is said in Koran 5:2] 'Help one another to good deeds and fear of Heaven, and don't help one another to sin and aggression. Fear Allah, for Allah punishes harshly.'
"Sheikh Al-Islam Ibn Taymiyya said: 'Whosoever thinks that churches are Allah's houses and serve as places for His worship, or whosoever thinks that the deeds of the Jews and the Christians are worship of Allah and obedience to His Prophet, and whosoever likes this and permits it or helps them [the unbelievers] to open [houses of worship] and to perform their religion and thinks this to be proximity or obedience [to Allah] - he is an unbeliever.'
"He also said: 'Whosoever thinks that visiting dhimmis [monotheist non-Muslims under Muslim rule] in their churches is proximity to Allah, he is an apostate. If he didn't know that this was forbidden, he should be so informed, and then if he persists, he is an apostate.'
"We find refuge in Allah in order not to backtrack from the right path… Those who turned back on their tracks after the right path was clear to them - Satan seduced them and filled their hearts with false hopes [Koran 47:25]'; 'They said to those who hated what Allah revealed: we will obey you in some matters, but Allah knows your secrets [Koran 47:26]'; 'How will it be when the angels take their souls and strike them on their faces and their backsides [Koran 47:27]'; 'This is because they followed that which angered Allah and they hated Allah's satisfaction, so he thwarts their actions [Koran 47:28]'.