Both biblical knowledge and secular knowledge are required.
…to bring into the king’s service some of the Israelites from the royal family and the nobility—young men without any physical defect, handsome, showing aptitude for every kind of learning, well informed, quick to understand, and qualified to serve in the king’s palace. He was to teach them the language and literature of the Babylonians…the king talked with them, and he found none equal to Daniel, Hananiah, Mishael and Azariah; so they entered the king’s service. In every matter of wisdom and understanding about which the king questioned them, he found them ten times better than all the magicians and enchanters in his whole kingdom. (Daniel 1:3-4, 19-20)
It requires deep thinking to identify an author’s worldview and then to wrestle with material. Often parents want to shelter their students from many aspects of secular culture, but only by understanding the culture can we produce students who are resilient to it and who are able to witness to others in it. With a firm biblical foundation, Daniel and his colleagues were subsequently steeped in the literature and knowledge of the Babylonians, and they were used by God mightily. Not that students need to be exposed to salacious or lewd material; emphatically, no! Nevertheless, students who are able to read and grasp secular or contrary works and then garner a working understanding will be better able to articulate their own faith and have a deeper appreciation for those who are lost. We cannot expect to be fishers of men if we cannot get close to the water.