Even if the IRS takes away a church's exemption letter, there is already case law that the church would still retain its tax-exempt status. There is a difference between the tax-exempt letter being sent back, but still having a status of tax-exempt, at least when it comes to churches. They lose their letter, not their status. It's sounds funny but that is what it is.
Churches pre-date the country, they have always been tax exempt and will continue to be.
Now christian groups that are not churches, if they lose their tax exempt letter, then they have truly lost their tax exempt status. It works this way for non-church religious groups because they are not a church.
That is the way it is, but I am afraid there are courts today that would not rule this way. Church tax-exemption was always considered a Constitutionally protected right. LBJ created a law to try to turn that Consitutionally protected right into a right granted by law. Now even many conservatives think government should have control over church's tax-exempt status, which in effect gives the government control over the establishment of religion.