Question

Which book became the core of calvinism

Answer 1

For Calvinists, the most important book is the Bible. The main book that differentiates Calvinism from other branches of christianity, is Calvin's book "Institutes of the Christian Religion" (original title: Institutio Christianae Religionis, published in 1536). hope this helps

Answer 2

The book Institutes of the Christian Religion became the core of Calvinism.