Mine Is the Only Truth is a common Ongoing Scheme card from Duskmourn: House of Horror Commander (DSC), collector number 340. Current market price: $0.33, TCGplayer market prices via Scryfall.
(An ongoing scheme remains face up until it's abandoned.) Whenever a player casts a spell, you draw a card. At the beginning of your upkeep, if you drew a card last turn, abandon this scheme.