Virgil, bucking up Dante as he flags near the end of their journey through Hell:

‘Now must you cast off sloth,’ my master said.
‘Sitting on feather cushions or stretched out
under comforters, no one comes to fame.

‘Without fame, he who spends his time on earth
leaves only such a mark upon the world
as smoke does on the air or foam on water.

‘Get to your feet! Conquer this laboring breath
with strength of mind, which wins the battle
if not dragged down by body’s weight.

‘There is a longer stair that must be climbed.
It’s not enough to leave these souls behind.
If you take my meaning, let it be of use.’

— Inferno XXIV, 46-57 (trans. Hollander)

You have to be careful with this. Virgil, though a good man, is not a Christian. For him, achieving immortality is about doing something to cause people to remember you long after you die. As a Christian work, the Commedia later makes it extremely clear that fame is itself transitory, and therefore a false ideal. We shouldn’t expect a virtuous pagan, even one as wise as Virgil, to share a Christian outlook. In fact, one of the key aspects of the poem is that Virgil represents the supreme incarnation of human Reason. Dante cannot achieve the heights of Heaven through Reason alone, which is why Virgil abandons him atop the mountain of Purgatory, and yields his role as guide to Beatrice, an allegory of Divine Revelation. As Robert Hollander says in his notes, if Thomas Aquinas had been Dante’s guide at this point in the journey, the advice would have been different.

Then again, Virgil knows that Dante has to carry on if he’s going to achieve his ultimate destination. Perhaps he knows that Dante, still in the first stage of his journey, is still too unrefined to respond to being told something along the lines of, “You should keep fighting because your reward will be unity with the Godhead.” Who drags himself up a cliff in Hell for something so abstract? Similarly, a platoon commander wouldn’t get very far in the heat of battle, telling his men that they need to keep pushing forward to take the next ridge because the fate of democracy depends on their action. He has to speak to their more immediate situation, and motivate them with a lower goal.

Another example: let’s say a priest is present in a convenience store when a thug pulls out a gun and attempts an armed robbery. In that moment, the thief is not likely to be motivated to stand down by an appeal to the ultimate good, which is the Love of God; he is far more likely to be motivated by the priest saying, “Son, listen to me! If you go through with this, you are on your way to Hell.” It’s not untrue, of course, but it’s a lower and more visceral truth that may be more accessible to the person in that stage of his life. In that sense, what Virgil tells Dante here should be read more broadly, as if to say, “No pain, no gain, Dante. If you’re going to amount to anything, you have to be willing to work for it.”

Dante is physically weary from the struggle through Hell, and doubts his strength to carry on. But Virgil is telling him that simply turning away from embracing sin is not sufficient. The penitent, if he would see God, has to be purged of the desire to sin. The souls in Purgatory do not have to worry about damnation, because they were penitent on earth, however imperfectly. God has saved them. But they will not be strong enough to bear the intensity of God’s love in Paradise until they have been purged, through asceticism, of the innate tendency to sin. This, Virgil says, is a harder task than simply ceasing to sin. Virgil says, in effect, “You think this is hard? You have no idea how much harder it’s going to get. But if you are going to amount to anything, and not just be one of the herd of lazy sheeple carried along to his fate. You are made of better stuff than that. You have what it takes inside you to overcome.”