The notion that there can be a science of divine things does not negate Thomas’ statements regarding how we are unable to know the essence of God. A science of divine things is possible so long as we are clear regarding what we actually can and cannot know, and how the science of divine things proceeds.
First of all, only God can know his own essence. In fact, he knows himself “through his essence” (Thomas Aquinas: Selected Writings, Penguin Books., London 1998. p. 116). In other words, only God can truly know himself, as it is his essence to do so. The science of divine things must proceed from this basic understanding, namely, we cannot essentially know God. This does not mean, however, that we cannot in any way come to know God (as we will see).
Secondly, in terms of human reason, according to Thomas we can approach knowledge of God referring to the fact of his existence (and not of what he is). This can be done in three ways: 1) observing his effects in creation; 2) understanding his causality of “more noble effects” which grants a “better display of his eminence”, and; 3) in a negative sense we understand more clearly how he transcends all things and defies definition (Thomas quotes Dionysius, “he is known as the cause, the excess and negation of all things” – (Aquinas, Selected Writings, p. 117).