Were people in the "Middle Ages" concerned with preparing for the afterlife over everything else? Or, put another way, how radical was the humanist idea that the world of the living was as important as that of the dead?

I wouldn’t go so far as to say that they were concerned with preparing for the afterlife “over everything else”—medieval people could be quite riotously earthly—but it was an important fixation, at least judging from the moral literature and art of the period. Images of the Last Judgment confronted them in practically every church they saw. It was frequently painted on the walls of their parish churches: Christ enthroned as a judge, the saved at his right hand being lifted to heaven, the dead at his left being dragged into the fiery mouth of hell. Christ sitting in judgment over the living and the dead—as the Apostles’ Creed promised—was sculpted in the tympanum over the west door, the main entrance door, of almost every major cathedral or monastic church. People saw these images all the time. The idea of making a “good death,” i.e., one with a priest at your side to hear your last confession and give you the eucharist as your “viaticum” (literally provisions for a journey) was an idea drummed into ordinary people and, insofar as spotty evidence suggests, was taken seriously. The most popular image painted in churches was that of St. Christopher carrying the Christ-child. Seeing it meant that you wouldn’t die an unexpected death that day. People certainly often enough debated points about death, especially in regions where heresy was considered a problem. How big were souls? If they were too big, wouldn’t they overflow the earth? At the resurrection of the flesh, did people come back with the same bodies they were buried with? Perhaps most pressing of all was the idea, well in place by the 1200s, of purgatory, where almost everyone was expected to spend time atoning for unfulfilled penance before going to heaven. There are countless stories in sermon literature of dead spirits coming back to plead with loved ones to offer prayers or masses to liberate them from purgatory. In fact, most medieval ghosts are souls in purgatory begging aid from the living.

On the other hand, the humanism that emerged in the long twelfth century (c. mid- to late- 1000s to early 1200s), the so-called “twelfth century Renaissance,” reoriented a lot of thinking about the natural world. The celebration of reason as God’s greatest gift to humanity meant that it should be used to understand the natural world, itself a gift from God. The world was comprehensible and knowledge of it could be systematized. It wasn’t to be scorned but explored as a thing filled with signs of the divine. You were not to be ensnared by it, but you also weren’t supposed to regard it as corrupt. This celebration of the natural world finds popular expression in the outlook of St. Francis, who saw all physical reality as a sign of God. See his famous [Canticle of the Sun] (http://www.franciscanfriarstor.com/archive/stfrancis/stf_canticle_of_the_sun.htm). The enhanced realism of early Renaissance painters such as Giotto (who painted the life cycle of St. Francis in his church at Assisi) reflect this newfound interest in the natural world. The Italian Renaissance built on this legacy but it didn’t not create it from scratch.

Sources: For the general outlook of medieval Christianity, Bernard Hamilton’s Religion in the Medieval West is an excellent short (190 pp) guide. For twelfth-century humanism, Richard Southern’s Scholastic Humanism and the Unification of Europe is a massive (2 vols) and massively important resource.