Why can't heretics be theologians? To think theologically is an exercise of reason reflecting on the data of divine Revelation for the purpose of understanding what is already believed by grace through faith. Heretics may be brilliant scholars, but because there is a defect in their faith, they cannot be true theologians. Here's why.
Sacred theology is a unique human science which proceeds from what is known by faith in divine Revelation rather than by what is discovered through investigation or experimentation, and for that reason, theological method is different from the method of any other science. But all human attempts to know and understand anything, including God, must proceed according to the nature and structure of human reason. Therefore, the laws which govern all reasoning must be rigorously maintained to ensure the intellectual integrity of theology. Laws of reason such as the Principle of Identity and the Principle of Non-Contradiction are indispensable for the operation of right reason (ratio recta) without which rational discourse on any subject is impossible. That theology is a science in which reason is not sovereign or autonomous does not mean that theology can proceed without reason. To think theologically it is first necessary to think and to think rigorously.
To Think Theologically
The science of sacred theology (as distinct from natural theology or metaphysics) began to emerge as an ordered body of knowledge in the first century after Christ, and the classical definition of the project was given in the eleventh century by St. Anselm of Canterbury: fides quaerens intellectum, faith seeking understanding. Sacred theology is, therefore, an attempt to understand what is first confessed as true because of the authority of God Who reveals Himself in Sacred Scripture and Sacred Tradition, which together constitute the single divine Deposit of Faith entrusted by Christ to His Church. Sacred theology begins with the acceptance of the Gospel by faith and must be referred to the Gospel at every point in order to ensure that the activity of reason is faithful to the data of divine Revelation.
Sacred Theology is Not Religious Studies
That theology begins with obedience of faith to the Gospel means, among other things, that unbelievers or heretics cannot be true theologians; only faithful disciples of the Lord Jesus can be true theologians. The personal beliefs of a professor of religious studies are theoretically of no greater significance to his science than are those of a biologist studying paramecia. The beliefs of a theologian, however, are absolutely essential and intrinsic to the integrity of his science, which is truly a sacred science the practice of which requires the theological virtue of faith and fidelity to the inspired and infallible Word of God.
The Role of Authority in Sacred Theology
Because Christ entrusted His Apostles with the sacred power to teach, sanctify, and govern in His place, the apostolic authority of the bishops who succeed the Apostles is intrinsic to the science of sacred theology. It is bishops, not theologians, who insure that the Gospel is transmitted authentically and authoritatively from generation to generation. Only after receiving the Gospel from the College of Bishops and its head (the Bishop of Rome) can theologians begin their enterprise of reflecting upon the data of Revelation to understand revealed truth more completely. For this reason, the exercise of episcopal authority in guaranteeing the content of the Gospel is not hostile or alien to the work of theologians. Indeed, the exercise of episcopal authority is the sine qua non of the possibility of sacred theology as a science distinct from metaphysics, and theologians are called to the same liberating obedience of faith which is the vocation of every Christian.
Sentire Cum Ecclesia
To be a theologian, in sum, requires that one be both a faithful disciple of Christ and a scholar prepared to think with the Church. Neither piety nor learning is sufficient to make one a theologian, but no one can be a true theologian without both genuine piety and deep learning.