The 'Summa Theologica' is a comprehensive work of Christian theology written by St. Thomas Aquinas in the 13th century, intended to explain the principles of faith and reason. It is structured in a question-and-answer format, addressing key theological concepts and issues, making it a cornerstone of Scholastic thought. This work played a pivotal role in shaping medieval universities, where Aquinas's ideas became foundational in the study of philosophy and theology.