The extra calvinisticum "teaches that the Eternal Son of God, even after the Incarnation, was united to the human nature to form One Person but was not restricted to the flesh" (Willis, Calvin's Catholic Christology, p. 1). The term was created by Lutheran theologians after hearing the Reformed argue that the Son's existence "beyond the flesh" did not pose a problem for the unity of the divine and human natures.
- E. D. Willis, _Calvin's Catholic Christology: The Function of the So-Called _Extra Calvinisticum in Calvin's Theology. Studies in Medieval and Reformation Thought, vol 2. Brill, 1966.
- Myk Habets, "Putting the 'extra' back into Calvinism," Scottish Journal of Theology 62.4 (2009): 441-456.