What were the interests of the humanists?

Humanism was an intellectual movement which is generally associated with the Italian Renaissance. Its emphasis was the humanistic arts as set forth by the classical Greeks and Romans. It did not, as most modern people assume, refer to a materialistic and non-theistic philosophical outlook. In fact, most of the Renaissance humanists were thoroughly Christian in their worldview, although they were more apt to criticize the Church than Christians in the middle ages.
As historian Peter Partner explains in Renaissance Rome, Portrait of a Society 1500–1559:

Humanism was not an ideological programme but a body of literary knowledge and linguistic skill based on the "revival of good letters", which was a revival of a late-antique philology and grammar.

The humanists believed that medieval scholars had corrupted Latin and the other humanities (such as literature), so they sought to restore it to its original beauty by "purifying" it according to the way it was used by Cicero and other classical authors.
Similarly, the humanists adopted a policy of ad fontes ("back to the sources") in most humanistic disciplines. By this, they meant that one should emphasize the primary sources of classical antiquity rather than commentaries or re-tellings. This resulted in new and better translations of classical works (including the Bible). The pursuit of ad fontes was made possible by a number of discoveries of primary source texts.