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.
For an excellent introductory guide to Aquinas follow this link.
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).
Thirdly, on top of our human reason, we may be “strengthened by a new illumination.” This new illumination does not undermine our human reason, but includes, “the light of faith” and “gifts of wisdom and understanding” by which we are able ourselves to transcend our natural reason in the contemplation of God. This contemplation is regarding his “immensity” in relation to what we can and do know, and not to be seen as knowledge of his essence (Aquinas, Selected Writings, p. 117).
A recognition of our limitations (limitations not in a bad sense but referring rather to the frame within which we know anything at all as creatures – like a sandbox limited by the edges of the box) and an acceptance of Divine aid are the two basic prerequisites for a science of divine things and obviously provide the basis for the methodology. Thomas explains that the science Divine things is composed of knowledge in two ways. The first way is in accordance with “our mode” by which we may know something of the Divine things (by way of negation – “Science treats of higher things principally by way of negation” ST I, q. 88, art. 2) from our knowledge of principles regarding sensible things (Aquinas, Selected Writings, p. 131). The second “mode” is that of the divine things themselves in which we may “share” and come to an increasing “likeness” by way of faith, though it is impossible for us to grasp the Divine things “in themselves” (i.e., essentially) in this life, (Aquinas, Selected Writings, p. 131). Our faith in this regard, like faith in any other regard, is oriented hopefully towards a future knowledge. Faith is kind of agitation or goad for the mind and heart by which we hopefully pursue God with a view to him granting us the satisfaction of what we desire, namely himself – the Beatific Vision.
Mind your Maker 