Humanist revival of Greek and Roman texts and the promotion of secular models for individual and political behavior became an apparent trend during the Renaissance. How does The Prince symbolize this change in European history? Please make clear connections to the book.

There are really two parts to this question. The first deals with the concept of humanism that gained sway during the Renaissance in Italy and beyond. The second addresses the related issue of secularism. Let us look at each part.
Machiavelli was a humanist through and through. Like most of his peers, he looked to antiquity for guidance and wisdom. The Prince, like all of his works, is full of classical references, allusions, and examples. In chapter 3, for example, he uses the history of the Roman Empire to demonstrate what happens when foreigners occupy a territory. In the next chapter, he points to Alexander the Great's rule over Persia in order to examine another aspect of the same issue. He then uses many other leaders from antiquity to demonstrate the uses of armed power. Like all humanists, Machiavelli used the past to inform the present. The Greeks and Romans offered a trove of examples that could be exploited for the benefit of Machiavelli's readers. 
When addressing the issue of secularism, Machiavelli's core message is that a ruler cannot be guided by abstract notions of morality, including Christian justice. Rather, he must make decisions based on what is most likely to preserve and expand his power. Machiavelli essentially argues that the ends (the preservation of power) justify the means (cruelty, treachery, or other methods). No message could be further from the idea of a "Christian king," who would govern according to God's will. As a result, Machiavelli's secularism was as pronounced as his reliance on classical texts.

The Prince promotes secularism in that Machiavelli states that a prince can retain power by promoting his own greatness.  He does not have to be good or seek God's favor, but he has to appear to be good—this gives the prince power and responsibility at the same time.  Machiavelli states that the prince should not be loved nor hated; he must be respected.  The prince must also act according to what the situation demands at the time.  The prince must also be a student of military strategy because most of the prince's subjects associate good leadership with public safety.  This is a significant departure from medieval thought, a time when warfare was often left up to God's will or the ability of individual soldiers.  Machiavelli gives the prince more decision-making power in defending his realm.  Machiavelli's text was quite controversial at the time because it minimized the role of religion in governance, but it has stood the test of time because of its lessons in leadership.