Believe it or not churches need money to operate. Salaries for staff. Utility bills to keep the lights and heat on. Budgets for missions abroad. Food programs for the needy. Outreach programs in the inner city. It all takes money,
Currently they are granted non-profit status precisely because of the work they do in their communities.
Take that status away and a good portion of their tithes would go to paying property taxes and income taxes, instead of what they are currently spending them on - charity.
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It seems obvious, but apparently not everyone realizes it.