Skepticism
Inherent in Descartes's philosophy is a deep-rooted, basic skepticism about the nature of reality. This skepticism, preceding postmodernism by centuries, is the core motivation for this set of investigative meditations, forcing Descartes to doubt everything from his senses and build reality up from the ground level. In this sort of radical skepticism, the only thing in whose existence Descartes can wholeheartedly believe is his own mind (cogito ergo sum, although the quotation is from A Discourse on Method). He battles against this skepticism throughout the book, finally emerging triumphant at the end with a coherent theory of metaphysics.
Dualism
There is a permeating dualism present through this work, one in which Descartes obviously believes: the division between mind and body (which would come to be known as Cartesian dualism). In his view, the mind is the only aspect of oneself that is verifiably existent, and its state of being is more real and true than any physical form (echoing a bit of Platonic philosophy there). There is a clean split between the intellectual/spiritual world and the physical world - the senses belong in the physical world, and Descartes argues that the only function of the senses is to allow a person to get around in this constricting world; real truth is found in rationality and in God.
God
After Descartes solidifies the logical existence of himself, his next step is to formalize the necessity of God's existence. He does so in the Third Meditation, although it seems that this belief has been present since the beginning of the work, albeit latently. Descartes sees God as the ultimate cause and end of humans, and his Christianity impacts every part of his philosophy, especially the pseudo-Platonic dualism (which has had a significant impact on the development of theology, one that might be construed as somewhat detrimental in some instances). Regardless, Descartes's religion is a major theme throughout this short work, and God is the only other main character in Descartes's reductionistic experiment.
