Ruling on a Muslim bequeathing money for building a church
Question

                I heard a fatwa from one of the shaykhs saying that it is permissible for a Muslim to leave instructions in his will that a church be built with his money after he dies and that this is not a sin. Is this correct?.
Praise be to Allah.We cannot imagine that any Muslim of sound mind who has knowledge of sharee’ah could issue a fatwa like this. There may have been a mistake in reporting it from him or in understanding what he said and so on. 
But we will answer on the basis of what is mentioned in the question. 
There is no difference of opinion among the Muslims that leaving a will to help people of sin is not permissible, because Allah says (interpretation of the meaning):
“…but do not help one another in sin and transgression. And fear Allaah…”
[al-Maa’idah 5:2]
We do not need to prove that houses of worship in which people other than Muslims worship are houses of sin because others are associated with Allah  in them and His Prophets are reviled and disbelieved, and the Qur'aan is reviled and mocked; Tawheed is opposed and shirk is supported and promoted; conspiracies are drawn up there against the believers in the Oneness of Allah. Haraam festivals are also held there, and alcohol is drunk, and immoral actions are sometimes committed. These matters are well known. 
Based on that, it is not at all permissible for a Muslim to leave a will stating that a church or any other non-Muslim place of worship should be built with his money. 
There is no difference of opinion concerning this ruling among the scholars. Hence we will quote just one opinion from each of the madhhabs of the well-known imams (Hanafi, Maaliki, Shaafa’i, Hanbali and Zaahiri): 
1.Hanafi madhhab
Al-Kasaani said in Badaa’i’ al-Sanaa’i’, 8/341: 
If a Muslim leaves a will to a synagogue or church, it is invalid, because it is a sin. End quote. 
2.Maaliki madhhab
It says in al-Mudawwanah (4/150): 
Ibn al-Qaasim was asked: Do you think it is permissible for a man to hire himself out to build a church, according to Maalik’s opinion? 
He said: It is not permissible for him to do that, because Maalik said: No man should hire himself out to do something that Allah has forbidden. 
Maalik said: He should not rent out or sell his house to someone who will use it as a church, and he should not rent out his mount to someone who will ride it to the church. End quote. 
3.Shaafa’i madhhab
Imam al-Shaafa’i said in al-Umm (4/225): 
If a Christian bequeaths more than one third of it and his heirs come to us, we will annul whatever exceeds one third if the heirs wish, just as we will annul it if the heirs of a Muslim wish. 
If he bequeaths one third of his wealth or part of it to build a church for the Christians to pray in or to rent it out be used as a church or for a church to be built in it or for land to be bought with it to be a charitable trust for the church and to support the church, and so on, then the bequest is invalid. End quote. 
4.Hanbali madhhab
It says in al-Mughni, 6/122: 
A bequest to commit sin or do something haraam is not valid, whether the one who leaves the bequest is a Muslim or a dhimmi. If he leaves a bequest for a church or Zoroastrian temple to be built or renovated or to spend on such places, it is invalid. This was the view of al-Shaafa’i. End quote. 
5.The zaahiri (literalist) madhhab
Ibn Hazm said in al-Muhalla (8/37): 
it is not permissible to leave a bequest for sinful purposes -whether that is done by a Muslim or by a kaafir - such as one who leaves a bequest to build a church and the like, because Allah says (interpretation of the meaning):
“…but do not help one another in sin and transgression. And fear Allaah…”
[al-Maa’idah 5:2]
“And so judge (you O Muhammad صلى الله عليه وسلم) among them by what Allaah has revealed”
[al-Maa’idah 5:49]
Whoever lets them carry out these instructions which are contrary to the ruling of Islam when he is able to prevent them, then he is helping them in sin and transgression.
End quote. 
These are the texts of the imams and some of them state the ruling quite clearly, which is that it is not allowed for the Christian to leave a bequest to a church -- even though he is a Christian -- so how about the ruling concerning a Muslim leaving a bequest to a church? 
Some of the scholars narrated that there was consensus on this point. 
Taqiy al-Deen al-Subki al-Shaafa’i (may Allah have mercy on him) said: 
Building a church is haraam according to consensus and the same applies to renovating it. This is what the fuqaha’ said: If a bequest is made to build a church, that bequest is invalid, because building a church is a sin, as is renovating it. It makes no difference whether the one leaving the bequest is a Muslim or a kaafir. End quote. 
 Fataawa al-Subki, 2/396 
Rather the matter is more serious than being a sin; building a place of worship for non-Muslims -- whether this is a church or anything else – implies love of kufr and spreading it, and fighting Tawheed, and encouraging disbelief in the Lord of the Worlds, and this may lead the one who does that to kufr. 
Imam Abu’l-Hasan al-Ash’ari (may Allah have mercy on him) said: 
Approving of kufr is kufr, and building a church in which Allah is disbelieved is kufr, because it is approving of kufr. End quote. 
Al-Furooq by al-Quraafi, 4/124 
After that, how can it be said that it is permissible for a Muslim to bequeath any of his wealth to build a church and that this is not a sin? “Glory be to You (O Allaah)! This is a great lie” [al-Noor 24:16]. 
We ask Allah, may He be exalted, to make Islam and the Muslims prevail and to bring the Muslims back to their religion. 
And Allah knows best.