Austin Gene Mayo, 20, was arrested in November, just days after the Virginia Dale Community Church was destroyed.
He pleaded guilty to arson on Friday and faces up to 12 years in prison and five years probation. Sentencing was set for April 2.
A passing motorist spotted the fire on Nov. 16 but firefighters could not stop the flames from destroying the building.
``I don't understand what would make someone want to do that,'' said Dana Moen-Wright, a church member who attended Friday's hearing.
She said reconstruction of the church was nearly complete. Donors gave nearly $90,000 for the project, according to congregation member Shirley Moen.
Mayo was a former Livermore Fire Protection District firefighter of the year.
The town of Virginia Dale is located about five miles south of the Wyoming state line.
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