Institutes of the Christian Religion (1559) - John Calvin
Book 2 - God the Redeemer
Chapter 8 - Explanation of the Moral Law:
The 10 Commandments
Sections 56-57 - Principles of the law - loving enemy
Section 56 - Evangelical counsels?
Loving and helping your enemy; not taking vengeance on him.
Prov 25:21; Ex 23:4-5; Heb 10:30; Deut 32:35; Lev 19:18.
The Medieval churchmen turned these commands into "counsels" - optional good behavior.
This is "pestilential ignorance."
Section 57 - To love our enemy is a genuine commandment
Matt 5:44-47; Luke 6:27-28, 32; Matt 18:17.
They dilute these by making them exhortations, or confining them to monks.
Augustine and Chrysostem and Gregory the Great didn't think so.
They say these are too hard, but it is harder to love God wholly.