I suppose Tertullian thought he was being clever, coining such an ironic expression of faith. After all, atheists like me, especially one whose mantra is “Ridenda Religio”, have condemned and ridiculed religion precisely for the absurdity of its beliefs and claims. To make absurdity itself a reason for believing (for example, in the Resurrection, “because it is impossible”, dixit Tertullian) is, to say the least provocative, but a sure way of grabbing attention.
St Augustine’s “Credo ut intelligam” – “I believe so that I may understand” – is a similar idea frequently used to establish (?) faith as a source, equally as valid as reason, for understanding. Recent Popes have rejected the “quia absurdum” (in fact “ineptum”, “incongruous”) as fideism, but adopted the “ut intelligam”. But it is just as . . . absurd. I can’t really comprehend that consecrated bread becomes Christic flesh, but I will understand if I believe it. C’mon ! You’ve got to be kidding ! The pity is I know you’re not.