This verse from roughly the ninth century:
Memento rerum conditor,
Nostri quod olim corporis
Sacrata ab alvo Virginis
Nascendo formam sumpseris.
exhorts the creator to remember that he took human form from the womb of a virgin. Since this is taken as a known fact, why is sumpseris (thou took) in the subjunctive mood? Why not the indicative sumpsisti?
A Primer of Ecclesiastical Latin, by John F. Collins, §135, “Indirect Statements: Object Clauses with Subjunctives”, offers:
Indirect statements in the form of object clauses introduced by quod, quia, quoniam ‘that’ may take the indicative; this is called the retained indicative. But the use of the subjunctive mood in such clauses emphasizes the grammatical subordination of the indirect statement:
Invēnimus quod Paulus Rōmam jam īsset.
‘We discovered that Paul had already gone to Rome.’
Crēdunt quia Chrīstus sit Dominus.
‘They believed that Christ is Lord.
Medieval Latin, 2nd. ed., by Harrington and Pucci, §7.10.1, “Mood in Indirect Discourse”, says:
Greek ὅτι is followed by the indicative. Quia and quod may be followed by either the indicative or the subjunctive; quod tends to take the subjunctive, quia the indicative.
Neither explanation really clarifies what is meant by choosing the subjunctive mood in the verse above. It does not seem to be indirect discourse. The subjunctive with quod does not appear to be purely a medieval phenomenon. Jerome translates Genesis 1.4 thus:
Et vidit Deus lucem quod esset bona et divisit lucem ac tenebras.
Deuteronomy 15.15 has both subjunctive and indicative subordinated to the same quod:
Memento quod et ipse servieris in terra Aegypti et liberavit te Dominus Deus…
So, what's the reason for the subjunctive here?
Does memento quod customarily take the subjunctive for some reason?
Does the indicative change the meaning or perhaps alter the shade of meaning?
Or is the custom regarding different moods in this context so weak that the mood makes no difference here?
Or is there something about the meaning of sumo here that calls for the subjunctive?
Or (the worst hypothesis of all) is sumpseris ungrammatical or nonsensical, chosen only because it rhymes?
If you could provide other examples of the choice of indicative or subjunctive in similar or contrasting circumstances, that might be especially enlightening. What I'm really hoping to understand is: What does memento quod sumpseris "echo" in the mind of a listener?