How did Chaucer's classical allusions (inclusions of Greek and Roman myths) and ekphrastic depictions of the temples in the Canterbury Tales spread knowledge of the classical world to his Christian, Middle-Ages audience?

Throughout The Canterbury Tales, and specifically in "The Knight's Tale" referenced in the question, Chaucer fills his narrative with classical allusions and detailed descriptions, including the ekphrasis of Theseus's Temples.
The use of allusions, both biblical and classical, function here, as is typical in medieval literature, as something of a medieval hypertext. Recognition of the allusion opens Chaucer's text to a dialogue with the prior text by which the educated reader might find points of comparison that complicate a simple reading. The Knight, for instance, alludes to ancient "matter of Thebes" romance material but filters it through his own Crusader and battle-scarred consciousness. For an audience less familiar with the classical tradition, Chaucer's anachronism similarly uses familiar concepts of medieval Christianity and knighthood to access the classical stories. The gap between the two worlds provides much of the irony and social commentary at work in the narrative.
This gap is evident as well in the ekphrasis developed for the Temples. Each of these Temples to a pagan deity creates an ironic portrait of what devotion to Mars, Venus, or Diana might entail. In each, the Temple adornment speaks to the difference between a lofty ideal—whether of military power, eros, or chastity—and the enactment of that ideal in the pagan world. These pagan allusions and the detailed descriptions of attributes associated with the apparently more morally ambiguous pagan gods offer the reader the delightful complexity of thought associated with the ancient texts. At the same time, these are contained within a morally more didactic medieval context that distances Chaucer from pagan values, even while he employs pagan literature. As in Chaucer's other tales, this gap in values between what is depicted and what the Christian medieval reader would expect challenges the reader to differentiate between the world of the text and the world of lived experience in which the reader dwells.