A growing number of people are turning to health-care ministries to cover their medical expenses instead of buying traditional insurance, a trend that could challenge the stability of the Affordable Care Act (ACA).

The ministries, which operate outside the insurance system and aren’t regulated by states, provide a health-care cost-sharing arrangement among people with similarly held beliefs. Their membership growth has been spurred by an ACA provision allowing participants in eligible ministries to avoid fines for not buying insurance.

Ministry officials estimate they have about 500,000 members nationwide, more than double the roughly 200,000 members before the law was enacted in 2010.