In response to Jon Quitslund's remark ('I wonder if the primary field of reference for this episode is not scriptural but secular'), is Spenser really thinking about the Aeneid -- especially the dreams and visions of Books IV and VI -- here? As the commentary tradition stresses, the central conflict for Aeneas is between attachment to the objects of his senses (and emotions) and obedience to the gods (which relies on his faith in the promises they have made him regarding the destiny of his descendants). But if we see this as the relevant intertext, Spenser's radically revising Virgil, and inverting the value system implied in the original -- because he's parallelling Redcrosse's abandonment of Una (the outcome of the episode, the result of Redcrosse's deception by Archimago) with Aeneas' abandonment of Dido (the result of his obedience to the gods, his belief in the message of Mercury). (Redcrosse and Una have been suggestively parallelled with Aeneas and Dido from the start -- the storm which drives them into the Wandring Wood reminding us of the storms in Books I and IV of the Aeneid, preparing us to see Redcrosse's desertion as a replay of Aeneas'.)
Mercury persuades Aeneas to exchange the tangible present good of his sojourn in Carthage for promised future 'regnum' in Italy -- a divine promise in which he must have that kind of 'faith' which James Nohrnberg was talking about (not a creed but an 'I-hope-to-know' referring to what God is yet to do). The fullest expression of that promise -- where it is presented to Aeneas' eyes, with the intention of dispelling any doubt ('et dubitamus...?' VI.806) -- is the vision of his future descendants shown him by Anchises in the underworld of Book VI. Spenser's House of Morpheus, from which Archimago's minion fetches the false dream, evokes Virgil's underworld in its 'double gates, ... / The one faire fram'd of burnisht Yvory, / The other all with siluer ouercast', and also I think in the simile 'a murmring winde, much like the sowne / Of swarming Bees', which recalls Aeneas' unborn descendants flitting about the Lethe 'ac veluti in pratis ubi apes aestate serena / floribus insidunt variis et candida circum / lilia funduntur, strepit omnis murmure campus' (707-9). The effect is to present the whole argument of Rome's divinely ordained destiny which underpins the Aeneid as a false imposition by a wicked sorceror, Aeneas' devotion to imperial destiny as a fall into idolatry as well as a breach of human faith (note that Redcrosse exchanges Una for Duessa, with her obvious associations with Rome).
Spenser's expanding hints of doubt already in Virgil. Mercury's embassy in Book IV is prompted by Rumor, 'singing truths and untruths equally' ('pariter facta atque infecta' IV.190). There's also the fact the his second appearance (in a dream) is only 'Mercurio similis', and Aeneas isn't sure it's really him, but obeys anyway (558, 577). And of course the underworld episode is framed by the elm full of somnia vana at the entrance and the twin gates of sleep (genimae somni portae) at the end, with Aeneas exiting (like Archimago's spirit) through the ivory one, intended for false dreams. Those hints in Virgil, and Spenser's whole episode, seem to endorse Dido's reaction to Aeneas' tale that what he's doing is obeying the express command of the gods -- sarcastic disbelief.
I think the upshot of all this is that Spenser's overturning Virgil's views on the value and relation to the divine of 1) sexual love and 2) worldly empire. In the terms of this discussion, where Virgil equates sexual love with slavery to the senses and religious faith with commitment to empire, Spenser's showng that at least sometimes slavery to the senses can really reside in imperial ambition, and (religious) faith in amatory fidelity.
Sorry for the long posting!