Let the title not mislead anyone in to believing that there are Churches in Saudi Arabia. Non-Muslims can not even take symbols and pictures, related to their faith, in to Saudi Arabia with out getting them confiscated at the air port.
And also Grand Mufti is not calling for such destruction of Churches based on his personal opinion. He is just following Sharia, derived from Koran and Sunna - sayings and traditions of prophet Mohammad.
Numerous traditions related to Prophet Mohammad from Islamic sources suggest that Prophet Mohammad wanted to do some thing similar. Read the below:
It has been narrated by 'Umar b. al-Khattib that he heard the Messenger of Allah (may peace be upon him) say: I will expel the Jews and Christians from the Arabian Peninsula and will not leave any but Muslim. (Sahih Muslim; 19:4366)
.......The Prophet on his death-bed, gave three orders saying, "Expel the pagans from the Arabian Peninsula, respect and give gifts to the foreign delegates as you have seen me dealing with them." I forgot the third (order)" .......(Sahih Bukhari; 4:52:288)
Yahya related to me from Malik from Ismail ibn Abi Hakim that he heard Umar ibn Abd al-Aziz say, "One of the last things that the Messenger of Allah, may Allah bless him and grant him peace, said was, 'May Allah fight the jews and the christians. They took the graves of their Prophets as places of prostration . Two deens shall not co-exist in the land of the Arabs.' " - (Malik's Muwatta, Book 45, Number 45.5.17)
Saudi Arabia does not have Churches but the contention is 'what are the lands of Arabs' or what region comprises Arabian Peninsula.
When Grand Mufti called for such action, we have not seen any kind of condemnation from Muslim spokesmen and their apologists insisting Islam is one peaceful and tolerant religion or any Islamic organization. But why do they have to do it? After all, neither infidel nations nor United Nations -which normally condemns every action deemed to be offensive to Muslims - said any thing.
But then again, we have accepted and got used to much worse things like ethnic cleansing of Non-Muslims from Islamic lands because Muslims are just being Muslims. But if we say anything that sheds light on Islam from Islamic sources, we are labelled Islamophobes.