Kierkegaard’s only valid form of certain, objective knowledge is in the realm of logic (or its extension of mathematics). Theoretically, approximate objective knowledge can be gained in the realm of history and science, but it’s only ever approximate, so it can’t be said to be “certain,” and is always subject to revision. M.G. Piety details SK’s distinct categories of knowledge in her book Ways of Knowing: Kierkegaard’s Pluralistic Epistemology, and those are the only two realms of objective knowledge she can distinctly identify.
Interestingly, McCombs addresses how this situation has reversed in contemporary culture in The Paradoxical Rationality of Søren Kierkegaard:
Although the Hegelian System is now dead and gone [what Kierkegaard spent much of his time lambasting], systematizing is still alive and well. For example, scientific naturalists claim to know that all reality is material reality, or that all reality is to be known, to the extent that it can be known, not in diverse ways but exclusively by the public methods of mathematical and empirical sciences. (70)
This is an inversion of SK’s notion of what counts as truly objective knowledge — math and logic provide objective knowledge according to Kierkegaard, but they are not the limit of what is knowable. This, in my opinion, is a large reason why we experience a large amount of doubt about any religious or ethical truth claims. We have accepted the cultural claim re: immanence — that we can only know what is provable empirically or logically because we cannot empirically or logically prove anything exists outside of the material world.