With Meditations on First Philosophy (published in 1647), Descartes revolutionised Western philosophy, having a profound impact on society from the sciences to the arts. As such, Meditations is said to be a seminal - and arguably the first - text that characterized the transition from Renaissance era philosophy to the Enlightenment. The work, diverted the prevailing religious Church doctrine of the time, by posing the question "what can I know to be certain". The results of this gave birth to perhaps Descartes' most famous phrase: "cogito ergo sum" or "I think therefore I am".
This question, and Descartes' attempt to answer it, formed the foundations for 17th Century philosophy and begun a series of polemics between philosophers of various schools, in particular the "Rationalists" and "Empiricists" who posed different epistemological ideas on how knowledge is aquired (through experience or reason, respectively). However, regardless of ones own philosophical standpoint, it is undeniable that Descartes triggered the debate in these essential ideas and is perhaps the most important philosopher in the modern Western philosophical canon.
To understand the impact of the book, it is important to address the pre-existing culture. Since the Greek philosophers there had been few major leaps in Western philosophy, and during the medieval period what was said to be 'true' was derived mainly from Church doctrine (supposedly what God said to be true). With Meditations, Descartes shifted this rhetoric. While he was a devout Christian and supposedly believed his meditations to have come from God, the content of his work moved truth from being something only in the hands of the divine, to a more human-centric concept of truth derived from reason. In some senses, because of Meditations humanity gained autonomy over philosophical ideas, with the very concept of 'truth' morphing.
This in turn provoked a reaction towards the established religious order; humanity moved away from the church and God characterising the age of enlightenment. Descartes skeptical thought, from the Cartesian Circle to the Wax Argument, encouraging the reader to question what they hold to be true. It is arguably because of this that Meditations remains a key text for any philosophy student today.