"This mountain's of such sort that climbing it is hardest at the start; but as we rise, the slope grows less unkind." The speaker is the Roman poet Virgil, Dante's companion and guide in scaling the lofty mountain of Purgatory in the second book of Dante's tour-de-force account of the afterlife, "The Divine Comedy." They've already paid a harrowing visit to the "Inferno." Now it's the turn of "Purgatorio," where souls who need purifying are cleansed from the stain of sin. After that -- heaven. Note the neat bit of catechesis (one of many) that Dante, via Virgil, slips in casually: The great mountain of Purgatory -- which the souls must climb -- gets easier the higher they go. It's…