The method by which Joachim arrived at his three-age scheme, which he called concordia, involved matching the generations recorded in Old Testament genealogies against corresponding figures and periods in the New Testament and Church history, on the theory that history's two halves, before and after Christ, unfold in a structured correspondence rather than an arbitrary sequence. Marjorie Reeves's Influence of Prophecy treats this method as painstaking and genuinely original rather than simple numerology, distinguishing Joachim from many of the cruder date-setting prophets who followed him.
McGinn notes that Joachim's own generation-counting pointed him toward a transition out of the Age of the Son and into the Age of the Holy Spirit occurring around the year 1260, a specific figure that later Joachimist writers seized on and popularized far more insistently than Joachim's own more careful and qualified presentation of it, treating a date he had reached through his concordia method as though it were a firm prophecy rather than one output of a complex and revisable calculation. Joachim worked this concordance out in enormous technical detail across the Liber de Concordia, aligning specific Old Testament figures, from the patriarchs through the kings, against specific New Testament and Church-historical counterparts, a labor Reeves treats as closer in spirit to a scholarly commentator's cross-referencing than to a street prophet's vague pronouncement, even though the underlying premise, that such a correspondence exists and reveals God's plan for history, is itself a matter of faith rather than a claim later scholarship could verify.