God, being the supremely powerful, all-knowing all-seeing force that created the world and everything in it, was central in most 17th century philosophies. When investigating the importance of god in Descartes’ philosophy, one must first understand the science of the times. The basic principle of mechanism is that everything in the universe could eventually be explained in terms of mechanical laws, and with these laws there is no free will. The universe moves like a complex clock, with everything pre-determined and in accordance with the laws of matter. Pierre Simon de Laplace stated "We may regard the present state of the universe as the effect of the past and the cause of the future. An intellect which at any given moment knew all of the forces that animate nature and the mutual positions of the beings that compose it, if this intellect were vast enough to submit the data to analysis, could condense into a single formula the movement of the greatest bodies of the universe and that of the lightest atom; for such an intellect nothing could be uncertain and the future just like the past would be present before its eyes." (http://en.wikipedia.org/wiki/Mechanism_%28philosophy%29)
Unfortunately for mechanism, the mind is impossible to define in mechanical terms, as it is not a corporeal substance that can be measured. Descartes responds to this problem with dualism, saying that the mind is a “thinking thing” which is the essence of himself. This “thing” doubts, believes, hopes and thinks, all the while existing only in a