The answer to this is a bit complicated (no surprise there) and not terribly convincing, but here it is:

First, Descartes has to prove that there is something out there other than ourselves.  He argues that we can prove this by the fact that we have all these sensations (the heat from a fire) for example, that we do not consciously cause.  (We feel the heat whether we want to or not, whether we think about it or not.)  Because we don't cause the sensations, something outside us must cause them.

Second, he must prove that the something that causes the sensations is material.  He says there are three options for what could be causing them: A) God, B) something material, C) something else that's not material but created, presumably by God.

His only argument for why B is true is that God is not a deceiver.  He says that God has given us a propensity to believe in coroporeal things.  Since God would not deceive us, therefore there are corporeal/material things out there that are causing the sensations we feel.