Lt. Col. Saad al-Rashud said the 40 were arrested Friday in the neighborhood Badeea. Their church, he said, contained crosses and was run by a Pakistani man who claimed to heal the sick. He allegedly was holding prayers, hearing confessions and distributing communion.
It is illegal to promote religions other than Islam in Saudi Arabia, the birthplace of Islam. There are no legal churches in the conservative kingdom, where members of other religions generally can practice their faith in their own homes, but not try to convert people or hold religious gatherings.
Authorities said those arrested with him were foreigners, but did not specify nationalities.
A conviction on proselytizing can result in a harsh prison sentence followed by deportation.
A variety of raids have been carried out in the capital lately, with al-Rashud saying that another recent bust netted about 20 southeast Asian men and women who had formed a prostitution ring.
Separately, two makeshift alcohol factories were found, a variety of drugs were seized and 13 common criminals landed in jail, he added.