Anselm's motto, Credo ut intelligam, marks well the distance that has been traversed since Tertullian's Credo quia absurdum est.
4 He adduces it as a sort of reductio ad absurdum of Christians who would model life and cult on Christ and his apostles, unencumbered by later church traditions.
This was at once his strength and his weakness: his strength, for as a professional pleader he had learned how to deal with an adversary according to the rules of the art - to pull to pieces his theses, to reduce him ad absurdum, and to show the defects and contradictions of his statements, - and was specially qualified to expose the irregularities in the proceedings taken by the state against the Christians; but it was also his weakness, for it was responsible for his litigiousness, his often doubtful shifts and artifices, his sophisms and argumentationes ad hominem, his fallacies and surprises.
As a human person, I am body and soul; and the idealistic identification of the Ego with soul or mind, involving the corollary that my body belongs to the non-Ego and is no part of myself, is the reductio ad absurdum of idealism.
Credo, quia absurdum was applied, notably by the popular writers of the French Second Empire, in a fashion grotesquely literal enough to scandalize Tertullian himself.