For our purposes, “certainty” means subjective thought, thinking, or ideas, whereas “truth” refers to external reality, formalism, or the “noumenon” (to borrow a Kantian term). Thus we’re considering the relationship that exists between subject and object, as well as the various conceptions of that relationship that have emerged throughout the course of Western history. At the risk of oversimplification, the ancient and Medieval view – with rare exceptions – presupposes an immediate identity for certainty and truth: a thinking subject thinks of the truth, passively mirroring it, since the idea is identical to the reality.
Only with pre-Kantian modernity (14th to 18th century) did the “certainty vs. truth” dichotomy begin to come into play: certainty becomes deformed truth, derived from it, a compromise born of the action carried out by the subject. At any rate, the conviction remains that it is possible to discover the truth starting from either reason (rationalism – e.g., Descartes, Spinoza, Leibniz) or experience (empiricism – e.g., Locke, Hume, Berkeley).
A similar realization is rejected by Immanuel Kant, who will take the aforementioned opposition to an extreme, maintaining the existence of an unbridgeable gap between thinking and knowing: the object in itself is ineffable in so much as, once thought of, it loses its primigenial nature having been processed through intellectual faculties.
Conversely, the thought-knowledge dichotomy disappears with idealism: for Georg Wilhelm Friedrich Hegel and his compatriots, the two concepts intersect at the point where truth and certainty once again coincide. This time, however, the relationship is no longer immediate (the former position), but rather mediated by an active subject who produces everything and appears to be an actual creator of reality.
As Johann G. Fichte expressed it: The Ego posits itself as Ego in opposition to the non-Ego.