Adam was mortal only as able to die, and immortal only as able not to die
In their view Adam in his first state was not mortal in the first sense, only in the second. There was no necessity at all that he die, and had he persevered in original righteousness he would never actually have died. Yet it was possible for him to die, as in fact happened after his fall. Through sin he and all his descendants became mortal in the first sense, because he brought on himself the necessity of dying.
In the same way, man in the state of innocence was not immortal in the first sense, for he was not such that he simply could not die. He was immortal in the second sense: he was able not to die, and had he stayed faithful in obeying God he would never have tasted death.
Latin
III. Priori modo, ex eorum mente, Adam in primo statu non fuit mortalis, sed tantum posteriori. Siquidem eum mori necesse nihil erat: nec si in originali justitia perseverasset, revera fuisset mortuus. Sed tamen contingere poterat ut moreretur, quemadmodum revera post lapsum illius contigit. Per peccatum autem & ipse, & tota ejus posteritas facta est mortalis priori modo, quia necessitatem moriendi incurrit. Similiter homo in statu innocentiae priori modo non erat immortalis; nec enim erat talis qui simpliciter mori non posset. Verum immortalis erat posteriori modo, siquidem poterat non mori, & fidelis manens in Dei obsequio mortem expertus non fuisset.