Humanism was a radical break from everything medieval. The enotes Study Guide on humanism and its historical context addresses the issue with:
The Renaissance constituted a major shift in focus from God to the human. It started in the middle of the fourteenth century, after the Black Death (plague, 1347–1377) killed almost one-third of the population of Europe. Although the economy suffered, the remaining population earned higher wages and quickly filled in the gaps in the market. A renewed interest in classical literature, language, and philosophy fed the intellectual movement of the Renaissance: Humanism. Humanism was responsible for raising man to a level of dignity and intellectual importance that actually threatened the viability of the Church. As humanists worked to integrate pagan classical philosophy with Christian, Jewish, and gnostic theology and mysticism, they developed the notion that man can achieve redemption through his faith, independent of the grace of God.
And the enotes Study Guide summary on humanism adds the following:
Humanism is an educational and cultural philosophy that began in the Renaissance when scholars rediscovered Greek and Roman classical philosophy and has as its guiding principle the essential dignity of man. Humanism was the intellectual movement that informed the Renaissance, although the term itself was not used to describe this discovery of man until the early nineteenth century. Humanist thinking came about as a response to the scholasticism of the universities. The Schoolmen, or scholastics, valued Aristotelian logic, which they used in their complicated method of defending the scriptures through disputation of isolated statements. Humanists accused the scholastics of sophistry and of distorting the truth by arguing philosophical phrases taken out of context. By contrast, humanists researched the historical context and lives of classical writers and focused on the moral and ethical content of the texts. Along with this shift came the concept that “Man is the measure of all things” (Pythagoras), which meant that now Man was the center of the universe in place of God. In turn, the study of man and human acts on Earth led humanists to feel justified in entering into the affairs of the world, rather than leading a life of monastic asceticism, as did the scholastics.
Sophistry, mentioned above, is subtly deceptive reasoning or argumentation.
Although humanism broke with much of church tradition, in the beginning, humanists meshed the Christian with the emphasis on the human, and formed what came to be called Christian humanism. In time, however, humanism came to be associated with secularism, and separated itself from supernaturalism.
Humanism eventually led to the idea of democracy, a liberal arts education, and an emphasis on science. In fact, much of the world you live in owes a debt to the first humanists; men like Erasmus, Petrarch, and Milton.