What is in Circle One?

As Dante is constructing his version of Hell, he uses Aquinas’s model to condemn his sinners. In Aquinas’s rendering, the first section of hell is the “Vestibule,” a place that is non-descript, which is appropo considering its denizens. Here one finds the non-believers and the indecisive, those who refuse to choose sides in the eternal war. Opportunists are also housed here; they are those who take advantage of the gullible.
Dante, however, begins with what Aquinas identifies as “Limbo” and calls it “Circle One.” Infants who die without the benefit of baptism are here; so too are the virtuous pagans. These are people, who, like Virgil himself, are not eligible for forgiveness because they lived prior to Christ’s sacrifice for mankind. Socrates, Homer, and Plato live here. While they have lived good lives, they cannot be in God’s grace, and therefore they exist “without hope” and forever “In desire.”
Virgil explains to Dante:

"The anguish of the people
Who are below here in my face depicts
That pity which for terror thou hast taken.
Let us go on, for the long way impels us."
Thus he went in, and thus he made me enter
The foremost circle that surrounds the abyss.
There, as it seemed to me from listening,
Were lamentations none, but only sighs,
That tremble made the everlasting air.
And this arose from sorrow without torment,
Which the crowds had, that many were and great,
Of infants and of women and of men.
To me the Master good: "Thou dost not ask
What spirits these, which thou beholdest, are?
Now will I have thee know, ere thou go farther,
That they sinned not; and if they merit had,
'Tis not enough, because they had not baptism
Which is the portal of the Faith thou holdest;
And if they were before Christianity,
In the right manner they adored not God;
And among such as these am I myself.
For such defects, and not for other guilt,
Lost are we and are only so far punished,
That without hope we live on in desire."