In this book, Tertullian Defends, in all Essential Points, the Doctrine of the Holy Trinity, stating that Praxeas' beliefs are heresy:

"In various ways has the devil rivalled and resisted the truth. Sometimes his aim has been to destroy the truth by defending it. He maintains that there is one only Lord, the Almighty Creator of the world, in order that out of this doctrine of the unity he may fabricate a heresy."

Praxeas was a Monarchian from Asia Minor. He believed in the unity of the Godhead and vehemently disagreed with any attempt at division of the personalities or personages of the Father, Son, and Holy Spirit in the Christian Church.