More specifically:
* Theology: This was paramount, encompassing biblical studies, church history, and philosophical discussions related to faith. Becoming a clergyman was a very common path for graduates.
* Classics: This included the study of Greek and Latin literature, grammar, rhetoric, and philosophy. A deep understanding of these languages was crucial to accessing much of the existing knowledge at the time. This also encompassed history and philosophy derived from classical sources.
Other subjects emerged slowly over time, branching out from these core disciplines. The development of other fields was gradual and often intertwined with existing ones. For example, aspects of what we'd now call philosophy or law were incorporated within the classical and theological frameworks. Mathematics and some aspects of natural philosophy (a precursor to science) gradually gained ground, but remained secondary for centuries.