If you are mowing the church lawn with your own lawn mower, you can deduct your actual out of pocket expenses such as gas, and mileage from your home to the church, assuming you kept records. These would be business expenses on your schedule C. (If you don't have records of your expenses, and get audited, you will lose the deductions.)
If you were in the business of providing lawn care services and the church was just one client, and you had a business liability insurance policy, that would be a business expense. You could also take depreciation on the lawnmower. However, that isn't really practical if this is your only work (you are basically paid a small amount for volunteering). There's no way to assign a business percentage to your personal insurance policy or the minimal amount of business use for your personal lawnmower. Your only business expenses that are likely to survive an audit are your gas and mileage if you tracked it.
You would owe income tax plus self-employment tax (15%, social security and medicare for self-employed people) on the net profit (income after expenses).
If you don't have other work (are unemployed or retired), having a small amount of income earned from working can make you eligible to make an IRA contribution or do a few other beneficial things, so there are some potential advantages to reporting it this way.
Now, there is something you can try if you don't want to pay SE tax: Schedule C and self employment tax are required if you are in business but not for a hobby or "other income." If you are not really in business to mow lawns, but are basically being compensated a small amount for being a volunteer, you can report the income as "other income". Other income is not subject to SE tax; on the other hand, you can only deduct your expenses as miscellaneous itemized deductions subject to the 2% rule, which basically means you get no practical deduction for expenses. But you save the 15% SE tax. Assuming the church puts the income on line 7 of the 1099-MISC, both Turbotax and the IRS will assume it is self-employment, but you can get Turbotax to treat it as Other Income if you answer all the test questions as if this is not work income -- not similar to your regular work, not intended to earn a profit, not planning to do in the past or future. Then, you should expect that the IRS will send you a letter asking why you did not include a schedule C and pay SE tax. You would respond buy explaining the situation and hope they accept your answer -- you are not "in a trade or business", you are being compensated for occasional volunteer work, etc. See this article https://www.journalofaccountancy.com/issues/2009/jul/20091639.html
