Nobody really knows, because religious groups don’t need to follow regular accounting and disclosure rules.
Pope Francis puts flowers on the altar inside St. Mary Major Basilica
– How Rich Is the Catholic Church? (Slate, March 14, 2013):
Pope Francis is not just the spiritual leader of one of the world’s major religions: He’s also the head of what’s probably the wealthiest institution in the entire world. The Catholic Church’s global spending matches the annual revenues of the planet’s largest firms, and its assets—huge amounts of real estate, St. Patrick’s Cathedral, Vatican City, some of the world’s greatest art—surely exceed those of any corporation by an order of magnitude.*
But it turns out to be surprisingly difficult to understand exactly how rich the church is. That’s in part because church finances are complicated. But it’s also because, in the United States at least, churches in general are exempted from the financial reporting and disclosure requirements that otherwise apply to nonprofit groups. And it turns out, that exemption may have undesirable consequences.