The distinction, first, involves the point of view. From the point of view of theology, there are things that can be known by reason alone, and some of those things have been revealed by God, but they might not have been. These are things revealable.
They are encompassed by theology when they are investigated and understood in their relation to God as their source and their end. In addition, theology also involves those things that can be known only by revelation. For example, that God is a Trinity. These are the revelatum.
In this way, all things can be considered within the scope of theology because they are treated sub ratione Dei, under the logic of God, within the context of God being their source and end, and of the ordering of all things within a hierarchical structure of all reality towards God. For Aquinas, theology is a science of revelation, acquired from the Word of God. This is his idea of sacra doctrina – the sacred doctrine of God and of the beatorum.
From the point of view of philosophy, things that can be known apart from revelation can be extracted from their theological context and be viewed as they are in themselves, judged from the point of view of natural reason. In this way, not only the physical sciences, but also metaphysical knowledge, can be investigated in its own right, as well as be inserted in a theological structure without losing its strictly philosophical nature.
There is a science of reason that can be integrated with the science of revelation without compromising the structure of either (although, when integrated, the science of reason is subordinated to and ordered according to the science of revelation). Philosophy investigates creatures as they are in themselves, and theology investigates all things as they are in relation to God.
In this way, there are things that are accessible to human understanding and reason; for example, mathematics, physical sciences, metaphysical inquiry in which the existence of a simple Being whose essence is identical with its existence is the Prime Mover of all creation, etc.
These things are in principle knowable by human reason unaided by revelation, even though some of them have been revealed. They are the revealable, the revelabilia.
This human learning, then can be incorporated into theology. Some of these things have been given through revelation: for example, a metaphysician can, by reason alone, demonstrate the existence of God. But since not all men are metaphysicians – they do not have the training, the aptitude, the time, etc. – God has revealed that in order that all men might be saved.
There are also things that can be known only by revelation. This is the realm of the revelatum. Examples are the Trinity, the Incarnation, sin, etc. They are not accessible to human reason save by revelation; this is not to say they conflict with human reason, quite the contrary. But it is to say that human reason alone can neither attain, deduce, or prove those things.
Theology therefore depends solely on what is revealed by sacred scripture, and order all things – even things that are known by human reason alone, some of which have been revealed and some of which have not – sub ratione Dei.
Theology contains both what has been revealed by God and our rational understanding of that revelation. Theology receives what has been revealed and then spreads itself to consider all things under that point of view, ordered from God and to God. The things necessary for salvation are the articles of faith, which God has revealed.
Other things, which can be incorporated into theology, are not necessary for salvation, but they make the understanding of things necessary for salvation more explicit.
For Aquinas, then, sacred science can consider all things – all branches of philosophy – from one point of view, insofar as they are revealable, and ordered according to God as their source and end. Faith and reason can be viewed as two intersecting circles, because there are things that are included in both at the same time (things knowable by reason alone which have also been revealed). The circles also have their content which do not intersect – e.g., the Trinity belongs to the realm of theology and faith and it is not attainable or proven by reason (these are things necessary for salvation), and there are also things known by reason alone that God has not revealed, even if he could have.
Reason and faith therefore are neither put in opposition, or in isolation, or in identity.Each has its proper realm according to the point of view of how they are considered.
Theology, however, is the higher science because it can encompass everything philosophy can discover, but it also includes things revealed that philosophy cannot discover. Also, when theology consider all revealable things sub ratione Dei, it does so in a way that is in accordance with human reason, because human reason, while limited and unable to attain to the revelatum, is never in conflict with it. Even when faith and reason cover the same territory, they each retain their own characteristics.
For human reason, sensible objects are always the point of departure of all of our knowledge, even as they have retained vestiges of the divine nature as their cause. Reason thus can have a preliminary role of pointing us to investigate the Ultimate Cause. It can also have a didactic purpose of explaining what has been revealed: either in the different ways of apologetics (confuting Christian or pagan error, etc.) as well as developing Christian theology in a coherent and encompassing way, approximating the knowledge of all things to the knowledge God has of himself and the knowledge that the blessed in heaven have.
Reason is used by theology to clarify the truths of faith. But the truths of faith that depend of revelation – the revelatum – cannot be attainable by reason alone or proven by reason alone. The attempt to do so is to confirm the unbeliever in his unbelief.
In this way, Aquinas is neither a rationalist nor a fideist.
He is not a rationalist because he makes very clear that things that are necessary for salvation can only be known through revelation and received by faith. He is not a fideist because he both preserves the legitimate realm of philosophy in which things can be considered as they are in themselves, as well as incorporates it into the realm of theology in which all things are considered from the point of view of revelation, while being understood in accordance with human reason.
Aquinas protects theology from rationalism and creation from contemptio mundi. In this way, grace does not destroy nature but perfects it.