If you are self-employed, you have to pay a self-employment tax in April, which amounts to your contribution to the Social Security and Medicare trust funds. (If you worked for someone else, those contributions are automatically deducted from each paycheck and your employer covers half of the cost.) But the tax code recognizes that not everyone wants to collect Social Security or Medicare assistance when they retire, and therefore shouldn't have to pay into the system.
Rather than opening up this exemption to all tax objectors, the IRS limits the self-employment tax exemption to licensed and ordained clergy members, and members of certain religious orders who have conscientious objections to receiving social services from the government [source: IRS]. This exemption is often claimed by members of Amish and Mennonite communities and Christian Science practitioners.
To claim the self-employment tax exemption, you must apply to the IRS using Form 4361. God speed.