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It dates back to Mosaic Law; you were supposed to give the best 10% of your grain or livestock to God (IIRC, the Levite tribe, who were landless priests, supported themselves out of these and other offerings). In modern days, one is supposed to give at least a tenth of one's income to support God's Work; this tends to get twisted a little by some televangelists (see: begathon, John Avanzini) who try to coerce you into giving to them, promising immediate rewards - they quote Malachi 3:10 to back it up.