Andrew (S.), thanks for the book recommendation.
Later on in the letters, Pascal himself provided what (to him) seems to have been the point of heresy of the Calvinists: that they didn't believe in real presence, and didn't adore the elements of the sacrament. That didn't enter into the article Andrew (M.) linked to at all; which made me wonder... But for Pascal and the people at Port Royal, who seem to have been devoted to worshipping the elements of the communion, this was the real departure of Calvinists from the true faith, and the reason why "Calvinist" was such a reproach.
It was interesting in the article to sense the almost surprise of whoever was writing it, that Calvinists were not fatalists with regard to how they lived in the world. An A&E documentary about Christianity mentioned the same thing in the same sort of surprised way.