May asked:

What is Descartes's argument to show that God is not a deceiver?

In Meditation 4 Descartes describes the role of "understanding" and "will" in human judgements, in an
argument to show that God cannot be blamed for our mistakes. But I'm having trouble understanding
the two roles.

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"What is Descartes's argument to show that God is not a deceiver?"

I don't think that Descartes actually argues that God is not a deceiver since he just takes it for granted
that an infinitely good God would not deceive. But it might seem that God is a deceiver because we
may not have made the effort to achieve the clear and distinct ideas which God then guarantees are
true. But if we do our part in achieving knowledge, God will do his part, for he cannot deceive. But see
my reply to the second part of your question.

"In Meditation 4 Descartes describes the role of "understanding" and "will" in human judgements..."

According to Descartes, we persons are both similar to and also dissimilar to God.

We are similar to God in that our wills are as free as His is. This means that we have complete
freedom to assent or to dissent concerning our ideas. We are free, that is to say that an idea is true of
the world (assent); or that an idea is false (dissent). But we are dissimilar to God in that although his
understanding or intellect is infinite, ours is finite or limited. Thus, Descartes says, our wills are wider
than our intellect.

This combination of an infinitely free will, and a finite intellect leads to the possibility, and sometimes
the actuality of our making mistakes. For we can (and sometimes, do) assent to ideas when we have
not been able to make clear and distinct, or dissent from ideas that are in fact clear and distinct. If we
gave our assent to all and only our clear and distinct idea, we would never make any mistakes. But
we sometimes believe we know more than we actually do, or less than we actually do, so that we
make mistakes. So, for example, to us one of Descartes' examples, if I have a pain in my arm, I may
believe that the cause of the pain is also in my arm, but that goes beyond what I actually know, and
so, I may be mistaken because of the phenomenon of referred pain when damage elsewhere in my
body (a pinched nerve) may cause the pain in my arm. My will, my ability to assent to the proposition
that there is damage in my arm, went further than what I actually knew (how my physiology works) So
my mistake is my fault, and not God's. God gives me the ability to know the truth if I exactly proportion
my will to my intellect; but in my human pride, I do not do this.

Ken Stern