The Christian law is not forcing upon the passion of love something which is foreign to that passion’s own nature: it is demanding that lovers should take seriously something which their passion of itself impels them to do.
 C. S. Lewis, Mere Christianity (1952). copy citation

add
Author C. S. Lewis
Source Mere Christianity
Topic passion love
Date 1952
Language English
Reference
Note Adapted from a series of BBC radio talks made between 1942 and 1944
Weblink https://www.dacc.edu/assets/pdfs/PCM/merechristianitylewis.pdf

Context

“The curious thing is that lovers themselves, while they remain really in love, know this better than those who talk about love. As Chesterton pointed out, those who are in love have a natural inclination to bind themselves by promises. Love songs all over the world are full of vows of eternal constancy. The Christian law is not forcing upon the passion of love something which is foreign to that passion’s own nature: it is demanding that lovers should take seriously something which their passion of itself impels them to do. And, of course, the promise, made when I am in love and because I am in love, to be true to the beloved as long as I live, commits me to being true even if I cease to be in love. A promise must be about things that I can do, about actions: no one can promise to go on feeling in a certain way.” source