When discussing Descartes's methodological approach to truth and skepticism, it is important to be aware of the sheer extent to which human knowledge was being transformed by the Scientific Revolution at the time. If you look towards the medieval era, intellectual circles were primarily centered around the great classical thinkers and philosophers, along with Christian tradition and scripture. From this perspective, the Scientific Revolution well deserves the title revolutionary: for the early modern era, knowledge could be built through examination and experimentation, and thinkers of this era radically challenged the assumptions and received wisdom of the ancients and the church.
Descartes's attitude and appliance of skepticism was very much shaped by this intellectual context. But despite his employment of radical doubt, Descartes was not a skeptic. On the contrary, Descartes's purpose was to formulate a new philosophical system that could withstand the tests of skepticism. His purpose in methodologically testing all of his assumptions about existence was to find a single insight or idea that was irrefutable, and from that foundation, attempt to prove the existence of the external world and the validity of his intellectual assumptions and endeavors.
For Descartes, the core insight on which the rest of his system is built is the statement: "I think, therefore I am." He arrived at this point after implementing a series of increasingly radical tests of skeptical doubt, a thought experiment that culminates with his supposition that, instead of there being a benevolent God, God might instead be actively malicious, aiming to mislead him at every turn. But even in this most extreme case, Descartes holds that he cannot doubt that he himself exists (at least in some form), given that there must be a thinking being running this thought experiment to begin with. Even if this malicious cosmic deceiver exists, there still must be a mind deceived. With this claim, he has found a statement about the universe that is unquestionable, and he can use this as his foundation in creating a stronger, more enduring, philosophical system.