Who was he? Erasmus Desiderius of Rotterdam was an early Dutch humanist (in the Christian sense) a priest and a theologian who lived from probably 1467 (the year is disputed) until 1536. He is often credited with “laying the egg that Luther hatched” though that may be a bit of too close a relationship.
Why was he important? Erasmus basically founded the discipline of textual criticism and produced some of the first Greek editions of the New Testament that had been seen since the Western Church since its split from the Eastern Church (thanks, likely, to a combination of the renaissance and the crusades). He also criticized the Roman Catholic Church often using satire to do so, the most famous of which is In Praise of Folly. On the one hand he inspired Martin Luther and may have played a role in his life being spared early on in Luther’s conflict with the Roman church. On the other, he refused to leave the Roman Catholic Church, believing such a division was wrong, and was in harsh disagreement with Luther and other Reformers over whether or not we could call the will “free” believing that a free will was necessary. (Luther’s On the Bondage of the Will is, in some ways, a direct critique of Erasmus). Erasmus also believed in total religious toleration, and believed that external signs were nearly meaningless, instead believing that a person’s inward disposition toward God was what was important. In other words, he did not hold the sacraments up as entirely necessary. Because Erasmus refused to take sides during the reformation period, either for the Protestants or for the counter-reformation, he was, to an extent, vilified by both sides. In the end, though, he profoundly influenced both churches, most directly through his work with the New Testament.
Fun Fact: His last words were, “Dear God” and it appears that not only did he die without receiving last rites, but he made no move to even call a priest to administer them (in keeping with his de-emphasis on external signs). Despite having died a Roman Catholic, he was given a burial in Basel at the former cathedral which had by that point been taken over by the Swiss Reformed Church (thus mirroring, in some ways, his position between the two camps).
Where might I have heard of him? As I’ve already said, he is often credited with inspiring Martin Luther with the ideas of the Reformation, though Erasmus seems to have thought Luther went too far. Also he was one of the early humanists, who emphasized the role of intellectual study apart from a strictly theological sense, something that would come to full fruition in the Enlightenment.