Medieval scholars studied one of four fields at the university: theology, law, medicine, or the arts. The University of Paris was known to be the best concerning the instruction of theology. To many Medieval people, theology was the most respected discipline that a student could study in the university.
It cannot be underestimated how important theology was to many people in the Middle Ages. Indeed, many commentaries on the Bible were written in Medieval Times. Read Thomas Aquinas, Pope Innocent III, or Bonaventure---whether or not you agree with their interpretations of Scripture, it would be unimaginable to believe anyone claiming them to be ignorant of the Bible. They defended Christian doctrines as they understood them. Aquinas appealed not merely to philosophy but to Scripture as well. In his commentary on Romans, Aquinas did an amazing job of breaking Paul's ideas concerning justification. To Aquinas, Paul did not teach justification by faith alone as Luther would later believe. Instead, the ''Dumb Ox'' believed that Paul was distinguishing the works of the law from the law of faith (Romans 3: 27). Aquinas also held that justification is progressive---Catholic Christians, he believed continue to become justified before God.
When discussing theologians like Pope Innocent III or Thomas Aquinas, however, we must ask ourselves, what was at the core of their theology? Of course, they frequently quoted Ancient and Medieval fathers before them but the New Testament was essential to all that they believed (and the Old Testament as well). While Scripture in general, had aided much of the Medieval mind, the Gospel of Matthew played a significant influence on the thoughts of those who lived in the Middle Ages. In general, the medievals further developed many of the teachings found in the New Testament and Early Christianity---which brings me to my next point.
It's impossible to understand the Middle Ages without understanding Medieval Christianity and it's impossible to understand Medieval Christianity without studying the New Testament and Early Christianity. For example, historians should understand that the doctrines of the medievals concerning the debates over the filioque largely originated from Jesus's relationship to the Holy Spirit in the Gospel of John (John 14: 16, John 14: 26). Likewise, medievalists must be acquainted with Athanasius of Alexandria and his thoughts on the incarnation of Christ (C. S. Lewis stated that the Incarnation of Christ (John 1) was central to Medieval thought) in order to understand the theology of those in the Middle Ages. During the fourth century, when many in Christendom were challenging the doctrines of the Trinity, Athanasius elaborated on the Divinity of Christ in his work, On the Incarnation.
So what should medievalists study? The warfare, the siege weapons, the fashions, the daily lives of the people, etc. All of these are worthy fields for experts to engage much of their time. However, every medievalist should be familiar with both the Old and New Testaments as well as the history of Early Christianity in order for people to understand the teachings of the Latin Church, which had a profound influence on many in Christendom in the Post-Classical world.
