We do not get to guess or have to wonder at the theme or big idea of this letter. He states it up front. It is amazing you've left the gospel so quickly! Why do people leave? I think it is because the gospel of Jesus Christ opposes all that we naturally value.
The gospel did not originate with a few poeple thinking up a better religion. It was the revealed by God himself.
it is easy to miss, but Paul was not an instant, just add water (or a lightning bolt), apostle. He went to Arabia (some people think for as much as 8-9 years). Then he spent 3 years in Damascus. And, he knew the Old Testament by memory before he started this dozen or so years of training