How Abraham–Isaac–Jacob proves resurrection; why Christ argued from Moses
It is asked whether and how these words prove the resurrection. Those whose God is God, they live. But God is the God of Abraham, Isaac, and Jacob. Therefore Abraham, Isaac, and Jacob live. The major: there is a true and real relationship between God and them; if a true relation, there must be a true foundation on both sides. Therefore they cannot be utterly nothing, but truly exist in some way — certainly according to the soul if not according to the body. It would be the same if, proving the resurrection against the Sadducees, he proves the soul's immortality, because by proving this he proves that indirectly.
Secondly the sense is: they are not wholly withdrawn from the divine power, but so subject to it that they can be recalled to life at his good pleasure; Luke 20:38 hints this.
Why did Christ wish to prove by consequence when clear OT testimonies occur? Origen and Jerome note that the Sadducees received Moses alone. More rightly: since the Sadducees attacked the resurrection from Moses, Christ wished to confirm it from the same and to beat back their arrogance.
Latin
Quaeritur, An & quomodo haec verba resurrectionem probent. Quorum Deus est Deus, illi vivunt. Atqui Abrahami, Isaaci & Jacobi Deus est Deus. Ergo Abraham, Isaac & Jacob vivunt. Major: vera & realis relatio est inter Deum & ipsos; si vera relatio, verum fundamentum utrinque oportet. Non igitur prorsus nihil esse possunt, sed vere aliquo modo existunt. Vivunt certe secundum animam, si non secundum corpus. Perinde fuerit si resurrectionem adversus Sadducaeos probaturus animae immortalitatem probet, quia hanc probando illam probat indirecte. Secundo sensus: non penitus divinae potentiae subtracti sunt, sed ita ei subjacent ut pro ipsius beneplacito in vitam revocari possint; hoc Lucas v.38 innuit. Quaeritur, Cur Christus per consequentiam probare voluerit, cum in V.T. perspicua testimonia occurrant? Origenes & Hieronymus annotant Sadducaeos solum Mosen recepisse. Rectius: cum Sadducaei e Mose resurrectionem impugnarent, Christus voluit eam ex eodem confirmare, adversariorum arrogantiam retundere.