I have heard it more than once on Catholic radio that there is no such thing as a new heresy. Rather, old ones always appear in new clothes. But how on earth are we supposed to learn about them so as to defend ourselves against them?
St. Alphonsus Liguori, one of the greatest Doctors of the Church, has the answer in his magnificent book, The History of the Heresies and their Refutation. This 600 page book is an analysis of every heresy that the Church has faced since its beginnings at Pentecost. Throughout, St. Alphonsus writes with exacting detail about what the heretics believed and then refutes by using papal documents and the writings of the Saints.
If you want to learn about how to defend your faith against heresies then read this book! It is quite the investment!