The fact is certain because it is impossible.
Certum est quia impossibile est.
Original text (Latin)
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Source, in full
De Carne Christi, Chapter V, Part II. Called "Tertullian's rule of faith." Also given "Credo quia impossibile." I believe because it is impossible. Same idea in St. Augustine—Confessions, VI. 5. (7). Credo quia absurdum est. An anonymous rendering of the same.