What poem or poet is Nathaniel Hawthorne referring to in "Rappaccini's Daughter" when he writes, "The young stranger, who was not unstudied in the great poem of his country....."
The opening of "Rappaccini's Daughter" by Nathaniel Hawthorne states that
The young stranger, who was not unstudied in the great poem of his country, recollected that ... this family, ... had been pictured by Dante [in] ... his Inferno.
Thus, the end of the sentence from which you have quoted specifies that the "great poet" is Dante and the poem being referenced "The Inferno."
The significance of this opening is that the sufferers described in Dante's "Inferno" as sinners who were cast down into Hell (the "Inferno") of the poems title for sins so evil that Purgatory would not be adequate punishment. Having Giovanni Guasconti identify the armorial bearings of the house as one once possessed by a family mentioned by Dante does several things. First, it establishes Giovanni as well-educated and second it introduces an element of foreboding, warning us that there is some evil lurking in the house,which is confirmed by Professor Pietro Baglioni's warnings and the developments revealed later in the story.