The problem Thomas is addressing here is how people can know the essence of God, for, as he writes slightly earlier: “It is impossible to define the essence of God.” This alludes to what nowadays would be called apophatic theology, which is the acknowledgement that when one says anything about the essence of God one’s words will immediately fall short of the reality of God and so become inadequate. As Thomas says elsewhere: “What he is not is clearer to us than what he is.” Given this problem, Thomas proposes that we can know God by analogy with his effects in the same way as we can show in the sciences that there is a certain cause, shown in its effect, even though we cannot see the cause itself. This view, sometimes referred to as the analogia entis, is denied by the important Protestant theologian Karl Barth, who claimed that human beings are so sinful that they cannot know anything about God by his effects, and that the only way that God can teach us anything about him is through the direct intervention of his appearing in Christ from heaven.
Although we cannot know in what consists the essence of God, nevertheless in this science we make use of His effects, either of nature or of grace, in place of a definition, in regard to whatever is treated of in this science concerning God; even as in some philosophical sciences we demonstrate something about a cause from its effect, by taking the effect in place of a definition of the cause.
Thomsa Aquinas, Summa Theologica, [I, Q. 1, Art. 7]