IRS Publication 502 (2015)
What Medical Expenses Are Includible?
"You can include in medical expenses fees you pay to Christian Science practitioners for medical care."
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Q: What sort of medical care does a Christian Science practitioner provide?
A: Christian Science Practitioners practice a sort of argumentative prayer. The prayer argues that disease does not really exist, since God is good and would never have created anything that was not good. You have to pay for these prayers, but you can use pre-tax dollars to do it.
Q: What? Why would the IRS allow payments for prayer as an includible expense?
A: Childcare Is a Legal Duty (CHILD) explains how legislators dishonestly used anti-religious discrimination bills and pork barrel politics to promote payments to a religious cottage industry.
Adults may rely solely on prayer when they are sick because they are old enough to choose to live (or die) with the consequences. But they don't have that right on behalf of their minor children, because children are too young to assume the consequences, even if they think otherwise. Children can and do die or are disabled when their parents reject appropriate medical care on their behalf.
When public funds (Medicaid and Medicare*) pay for "spiritual healthcare" and the tax code permits taxpayers to use pre-tax dollars to pay for a Christian Science practitioner's prayers for their healing, the United States government is endorsing prayer, especially the prayer of Christian Science practitioners, as a legitimate form of medical care.
A government must serve the best interests of its citizens, especially those who are most vulnerable. When it suggests, implicitly or explicitly, that prayer is a legitimate form of medical care, it unwittingly supports parents in rejecting competent medical care for their children.
*The Affordable Care Act does not pay for prayers.