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In his method Descartes approaches the questions of the first philosophy, i.e. metaphysics, via the method of doubt. One could say that he has an epistemological approach, by first asking what he can know at all before doing actual metaphysics.
My question now is, whether he was the first one to do so.
If we invoke Descartes we should remember that he uses the Ontological Argument - or his own version of that protean argument. The original Ontological Argument as deployed by St Anselm (1077 or 1078 CE) also works from epistemology to metaphysics - from what we can conceive to what (necessarily) exists.
Anselm's exact argument - actually, pair of arguments - is a matter of scholarly dispute but it goes roughly like this :
(1) We have the concept of God, which is the concept of 'a-nature-than-which-no-greater-nature-can-be-thought'
(2) The concept of 'a-nature-than-which-no-greater-nature-can-be- thought' is intelligible.
(3) Hence we can take the concept of 'a-nature-than-which-no-greater-nature-can-be-thought' as instantiable. (There could be such a nature whether there actually is or not.)
(4) A nature which is instantiated in reality is greater than one which is not.
(5) So if a-nature-than-which-no-greater-nature-can-be-thought were not instantiated in reality, then it would be possible to think of a nature that is greater (for example, any nature that is in fact instantiated in reality).
(6) But this would be a contradiction, since it is obviously impossible to think of a nature that is greater than a-nature-than-which-no-greater-nature-can-be-thought.
(7) Therefore, a-nature-than-which-no-greater-nature-can-be-thought must indeed be instantiated in reality.
(8) Therefore, God is instantiated in reality, i.e. God exists.
I do not offer this, as presented, as an exercise in exact logic. (Getting Anselm into precise logical form is a huge task, no less because of textual than of formal difficulties.) But it represents a style of argument, preceding Descartes, that moves from epistemology to metaphysics : from concepts (the concept of God and of a-nature-than-which-no-greater-nature-can-be-thought) to reality (the existence of God).
Argument adapted and altered from Millican, 457-8; Nagasawa, 1029-30.
Endnote on St Augustine
One may add to the quotations cited in other answers :
For no one can "not know" that he himself is alive. If he is not alive, he cannot "not know" about it or anything else at all, because either to know or to "not know" implies a living subject. But, in such a case, by not positively affirming that they are alive, the skeptics ward off the appearance of error in themselves, yet they do make errors simply by showing themselves alive; one cannot err who is not alive. That we live is therefore not only true, but it is altogether certain as well. And there are many things that are thus true and certain concerning which, if we withhold positive assent, this ought not to be regarded as a higher wisdom but actually a sort of dementia. (Enchridon, VII.§ 20 : http://www.tertullian.org/fathers/augustine_enchiridion_02_trans.htm.
A version of the cogito appears in De Civitate Dei (On the City of God) :
'I am not at all afraid of the arguments of the Academicians, who say, ‘What if you are deceived?’ For if I am deceived, I am.' (De Civitate Dei, Book XI, 26). https://www.catholicstand.com/augustine-and-descartes-from-self-to-god/. [Nulla in his ueris Academicorum argumenta formido dicentium: Quid si falleris? Si enim fallor, sum : http://www.thelatinlibrary.com/augustine/civ11.shtml.]
References
Anselm, Proslogion, with the Replies of Gaunilo and Anselm, ISBN 10: 0872205657 / ISBN 13: 9780872205659 Published by Hackett Publishing Company, Inc., 2001.
Anselm, Proslogion, St Anselm : Basic Writings, trans. S.N. Deane, 2nd ed., La Salle : Open Court, 1962.
Peter Millican,'The One Fatal Flaw in Anselm's Argument'. Mind, 113, pp. 437-76.
Yujin Nagasawa, 'Millican on the Ontological Argument', Mind, New Series, Vol. 116, No. 464 (Oct., 2007), pp. 1027-1039.
One could say so, Descartes is often credited with initiating the epistemological turn:
"Modern philosophy is generally thought to be distinguished by an "epistemological turn." Prior philosophical tradition accorded special status to metaphysics, or "first philosophy" (the general philosophical investigation into the nature of reality). The modern tradition, by contrast, holds that it is necessary to determine the nature and bounds of human knowledge before any sure advance into metaphysics can be achieved."
Plato, Aristotle and scholastics, Descartes's direct predecessors, proceeded in the opposite direction: first laid down the ontology (more or less dogmatically), then explained how we can know what we know based on it. In other words, they put metaphysics first, hence its fame as the "first philosophy". Here is Lisska's characterization of Aquinas's epistemology in Thomas Aquinas on Phantasia, for example:
"I intend to show that Aquinas opts for a meta-philosophy totally at variance with what we find in modern philosophy. Aquinas builds his ontology first, and then his philosophy of mind and his epistemology follow from the ontological analysis already constructed. So, not only is Aquinas not a Cartesian in metaphysical dualism, but in a deeper sense, his approach to undertaking the activity of philosophy is worlds apart from the Cartesian method. There is, I suggest, a fundamental meta-philosophical difference between Aquinas and most practitioners of modern philosophy. In this, I am in agreement with Scott MacDonald who once wrote the following:"Aquinas does not build his philosophical system around a theory of knowledge. In fact, the reverse is true: he builds his epistemology on the basis provided by other parts of his system, in particular, his metaphysics and psychology"".
With the Cartesian distinction between objective things and subjective perceptions, free standing ideas distinct from impressions, etc., the possibility of "subjective idealism" or anti-realism was opened up, and explored by British empiricists, Kant and beyond. But one can find precursors to Descartes, St. Augustine is often named as one. Here is from Burnyeat's Idealism and Greek Philosophy: What Descartes Saw and Berkeley Missed:
"So far as I can discover, the first philosopher who picks out as something we know what are unambiguously subjective states, and picks them out as giving certain knowledge because they are subjective states, is Augustine (Contra Academicos III 26), in this as in other things a precursor of Descartes. It is clear that Augustine means to speak of subjective states, first because he uses verbs of appearance ("This appears white," "This tastes sweet," etc.), and second (in case anyone thinks to worry about a subjective reference for the demonstrative "this") because he has just invented the idea that we might designate as "the world" the totality of appearances, including the "as if" earth (quasi terra) and the "as if" sky which contains them (III 24). And Augustine thought of the claim to know items in this "world of appearance" not as a basis for skepticism, but as a novel way to refute the skeptical thesis that we have no knowledge of anything."
Augustine, of course, is also known for anticipating the Cogito to some extent. But even Burnyeat, all things considered, hands the priority over to Descartes:
"As with the Cogito, the Augustinian precedent does not amount to as much as one might expect. " Augustine claims knowledge of his own subjective states, because they are subjective states, but he does not give that knowledge a privileged status. The claim sits side by side with the claim that he knows simple logical and mathematical truths (Contr. Acad. III 21, 23, 25, 29), to which his ancient skeptical opponents had a ready reply (e.g., Cicero, Aca- demica 11 91-98), and with the claim that the skeptic himself must surely know whether he is a man or an ant (Contr. Acad. III 22), from Descartes' point of view an equally unpromising line of attack (cf. HR 1 150, 316-17). Whatever hints Augustine may have furnished, it was Descartes who put subjective knowledge at the center of epistemology - and thereby made idealism a possible position for a modern philosopher to take."
It would be better perhaps to say that Descartes was the first to achieve some success when approaching metaphysics epistemologically. Previous attempts were known to lead to skepticism and/or nihilism. Otherwise Gorgias who wrote On the non-existent is probably the first and some time after him skepticism became a whole school. Actually Descartes was fighting scepticism. 'Proving' the non-existence of the world cannot start without obvious self contradiction from the (metaphysical) assumption that it exists, so the conclusion and the starting ground are taken both to be epistemological.
Descartes was centuries late. The entire Perennial philosophy reduces to epistemology since 'knowing' would be fundamental. The idea that we can assess the limits of human understanding and knowledge without studying the nature of Reality would be considered idiotic.
