Understand that the church may reimburse/pay the volunteer whatever amount it deems prudent to pay for the costs. There is no law that tells the church what it must pay or that restricts or caps the pay to some specific amount.
The only real issue here is the tax effect of the reimbursement. To the extent that the church is simply reimbursing the volunteer for the actual cost the volunteer incurs carrying out church business I agree that is not taxable income to the volunteer, assuming that the church makes the employee account to the church for those costs. The employee would get no deduction on his federal income tax return for any costs that are covered by the reimbursement. Payment of expenses that are either not accounted for to the church or that are in excess of actual costs would result in taxable income to the volunteer. I'm inclined to agree with the CPA that the church may reimburse the 54 cents per mile in lieu of reimbursing actual expenses and the employee would not have taxable income from that, assuming the volunteer accounts to the church for the miles driven.

