The word theology, according to its etymology, means “teaching concerning God” (λόγος περὶ θεοῦ, de divinitate ratio sive sermo: St. Augustine, De civ. Dei VIII I). Thus theology is the science of God.
The material object of theology is firstly God, and secondly, created things under the aspect of their relation to God: Omnia pertractantur in sacra doctrina sub ratione Dei, vel quia sunt ipse Deus, vel quia habent ordinem ad Deum ut ad principium et finem. In sacred science all things are considered under the aspect of God, either because they are God Himself or because they refer to God as their beginning and end. S. th. I 1, 7.
As regards the Formal Object a distinction must be made between natural and supernatural theology. Natural theology was first expounded by Plato. It is called by St. Augustine, in agreement with Varro, Theologia Naturalis, and since the 19th century it is also called theodicy. It is the scientific exposition of the truths concerning God, in so far as these can be known by natural reason and thus may be regarded as the culmination of philosophy. Supernatural theology is the scientific exposition of the truths about God under the light of Divine Revelation. The formal object of natural theology is God, as He is known by natural reason from creation; the formal object of supernatural theology is God, as He is known by faith from revelation (cf. St. Augustine, De Civ. Dei VI 5: S. th. I 1, 1 ad 2).
Natural and supernatural theology differ: (a) in their principles of cognition, unaided human reason (ratio naturalis), reason illuminated by faith (ratio fide illustrata); (b) in their means of cognition, the study of created things (ea quae facta sunt), divine revelation (revelatio divina); (c) in their formal objects, God as Creator and Lord (Deus unus, Creator et Dominus), God one and three (Deus Unus et Trinus).