Oh- and who has arbitrarily decided that a fetus that apparently has no brain activity is not a human being?
Noone. It was concluded after very careful study of the development of fetuses in the womb.
If you believe in God, then you believe that God not only created a human being at the moment of conception, but even before:
That's just a belief though, and an unsupported one at that. The same source says prayer can heal all kinds of illnesses, but all evidence points to the contrary.
You have to understand that medicine is a science that in a very direct way deals with peoples lives. There is no room for superstition and fantasy - doctors must only look at what is real, not what they or their patients want to be real. That means that they can pay no heed to an ancient text and start claiming things by faith - real medicine requires a lot more work than that.
And as for no brain activity- maybe not detectable brain activity- but there must be something going on that is directing the multitude of cells to continue to grow and multiply, and ultimately to develop into a fully-formed baby. Surely no atheist believes that not only did life just accidently begin, but that even after a baby is conceived, the growth of that tiny human being into a fully-formed adult is due to a series of amazing accidents that just happen to all coincide to make the baby develop and grow (I jest, of course)?
Of course not. But that has nothing to do with brain activity. The growth of a baby (or any other organism) has nothing to do with brains - after all, most of the organisms on this planet don't have brains at all, and they are growing quite happily anyway. Even after birth, our growth has little to do with our brains. We mature and become adult whether we want to or not. It is not regulated by our brains.
As to what happens with the cells during the first trimester, well, they grow. Wikipedia has a nice summary, with sources,
here. There simply are not enough nerve cells to constitute a brain in the first trimester, and the cells that do exist are not yet connected. That happens in the second trimester.
I really don't understand your reasoning here- where did God ever suggest that we should execute anyone that has ever sinned, no matter what the crime.
Don't you get it? You claimed that pro-choice is an all-or-nothing stance. I used that very same logic to point out what happens when you apply it to other beliefs. Of course the bible doesn't claim that all crimes should be punished by death, just like pro-choice advocates don't claim that all abortions are okay - but you claimed they did or should. Do you see the error of your reasoning now?