Most people need to be restrained from immorality by fear of God and the afterlife. Atheists may be convinced of religion and virtue if rational philosophy answers questions of God and the soul. Descartes takes up Pope Leo X's challenge to Christian philosophers to convince skeptics that the soul survives the body.
An "evil spirit" might be deceiving us to think the world exists. Could everything be a dream?
Realize, however, that even in dreams, reason remains intact.
In essence, Descartes is not physical, but a thinking thing. If he stopped thinking, he would cease to exist.
Sensory perception and contemplation of other objects (in his example, a malleable piece of warm wax) help him comprehend how his mind works.
Does God exist and does he deceive?
A cause is greater than its effect, so only God could have caused Descartes to have the idea of an infinite being. His idea of God is prior to his idea of himself, "very clear and very distinct and contain[ing] more objective reality than does any other."
His mind wouldn't exist unless God first created it (his parents made only his body) and kept creating it anew at every moment. Anything that created him must have at least the idea of God's perfections, starting an infinite regress of more perfect beings. No regress is possible for his continual re-creation in the present moment.
One of God's perfections is that his perfections are unified, and the idea of this unification entails knowledge of all his perfections. He is not a deceiver.
God is no deceiver, having no imperfection of "weakness or malice."
God gave Descartes the ability to judge truth and falsehood. He can't know all God's purposes for him (e.g. whether it's better for him to be able to make mistakes), but God always wills the best.
Everything God has is greater than what he has (except free will, which has no degrees). Descartes is grateful that God created him with a few perfections. God chose not to give him the ability to know everything he needs, nor even to know when he doesn't know.
God's existence is part of his essence. A nonexistent God would be like a triangle without 180 degrees.
"Everything which I conceive clearly and distinctly is necessarily true," he wrote, because God doesn't deceive.
Physical things probably exist because the senses deliver truth more often than falsehood.
It is unknown why certain physical sensations correlate with psychological states (e.g. pain leads to unhappiness, hunger to a desire to eat). Unexplained sensory experiences like objects appearing smaller at a distance or phantom limb could cause Descartes to doubt his senses. Knowledge of an undeceiving God is needed for reassurance. God gives us faculties to be able to correct our false beliefs.
Rene Descartes. The Meditations Concerning First Philosophy. (1641) Published in Discourse on Method and Meditations. Translated by Laurence J. Lafleur. Indianapolis: Bobbs-Merrill Educational Publishing, 1960.
This summary was written in 2005, along with a series of other 500-word summaries of philosophy books, as an exercise in brevity.