In this canto Dante addresses one of the great moral problems of Christianity, which was particularly pressing for Renaissance scholars who revered the Ancients. Baptism is necessary for salvation, but it seems essentially unfair that all the good people who lived before Christianity, or who never heard of it, should suffer for something over which they had no control. Dante solves this problem by keeping the good Pagans and infidels in Hell, but giving them a painless and honorable fate. Limbo is not a happy place, but it is contemplative and calm. Its inhabitants are not tormented and they can converse with one another among green fields and noble castles.
The Great Lord is Christ, and his coming to Limbo is the harrowing of Hell, which in Christian teaching occurred after the crucifixion, when the good people of the Old Testament were posthumously saved.
Dante modestly pays himself a great compliment by having the great authors of Classical times accept him as one of them. Readers of the Inferno were presumably supposed to agree with these noble shades. It is important to notice that, according to Dante, no literature of importance had been written since Antiquity before Dante's work. This was a sentiment shared by many Renaissance writers, who ignored the medieval period and saw themselves as the direct heirs of the great Classical tradition.
The veneration of Aristotle is not accidental. In Dante's time, Aristotle was commonly referred as The Philosopher, the fount of all wisdom. The scholastic tradition of philosophy and theology, which was very powerful throughout the Renaissance period, is specifically that which united Aristotelian thought and Christian beliefs. Late medieval and Renaissance thinkers had a great deal of respect for received knowledge and the printed word, perhaps partly because there were so few books. Thus, although they were Christian, they were often unable to conceive of a system of knowledge which did not derive at least partially from the Ancients. After Dante's time, a rival group began to emerge, made up of those who preferred Plato to Aristotle. The fact that Dante was an Aristotelian is one reason he is often classified as a medieval poet rather than one of the Renaissance. Of course these classifications, though useful, are generally arbitrary: some historians strongly dispute the idea that time can be divided into specific periods.