why was renaissance humanism such a threat to the church?

http://en.wikipedia.org/wiki/Renaissance

Read the overview carefully. Make sure you also understand what the Middle Ages were about in order to understand how the Renaissance was different.
http://en.wikipedia.org/wiki/Middle_ages

Renaissance humanism posed a significant threat to the church during the Renaissance period for a few reasons. To understand why, let's first define Renaissance humanism. Renaissance humanism was an intellectual and cultural movement that emerged in Europe during the 14th to 16th centuries. It emphasized the study of classical knowledge, literature, and human-centered values such as individualism, reason, and secularism.

1. Focus on Human Potential: Renaissance humanism placed a strong emphasis on the potential of human beings to reason, create, and influence the world. This challenged the church's authority as the exclusive source of knowledge and power. The church promoted the idea of humans being inherently flawed and in need of salvation, whereas humanism celebrated human capabilities and achievements.

2. Critique of Clerical Corruption: Humanists were critical of the corrupt practices within the church, such as simony (buying and selling of religious positions) and nepotism (favoritism shown to relatives). They advocated for reform within the church hierarchy, expressing the need for a return to moral and ethical principles. These criticisms threatened the church's power and authority.

3. Conflict between Reason and Faith: Renaissance humanists stressed the importance of reason and observation in understanding the world, which sometimes conflicted with religious dogma and faith-based beliefs. They were interested in exploring the natural world and sought to understand it through empirical investigation and logic. This approach challenged certain religious doctrines that were based on faith alone, which caused tension with the church.

4. Promotion of Secularism: Renaissance humanists encouraged the study of secular subjects such as literature, history, and science. They believed in the value of this knowledge for personal development and societal progress. This emphasis on secular knowledge challenged the church's monopoly on education and knowledge, which threatened its control over people's beliefs and worldview.

It's important to note that the church did not view Renaissance humanism as a unified threat, but rather saw individual humanists and their ideas as potential challenges to its authority. The church responded to these perceived threats through various means, including censorship, persecution, and the establishment of the Inquisition to suppress dissenting views. Nevertheless, the ideas and values of Renaissance humanism had a lasting impact on European society and laid the groundwork for the Enlightenment and other later movements.