This statement assumes that the "medieval mind" held ideas that were very different from the ideas espoused by the humanists. By bringing their new ideas into circulation, the humanists made it much less possible to hold to a medieval mindset.
Specifically, we are told that people in medieval times were very centered on God. They did not think about the importance of human beings. They did not look for explanations as to why things happened beyond just saying that things were God's will. When the humanists came along, these ideas were challenged.
The humanists argued for the inherent importance of human beings. They argued for the idea that the world could be understood through logic and science. These ideas went directly against the medieval mindset and, therefore, "shattered" it.