No man knows how bad he is till he has tried very hard to be good. A silly idea is current that good people do not know what temptation means.
 C. S. Lewis, Mere Christianity (1952). copy citation

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Author C. S. Lewis
Source Mere Christianity
Topic temptation meaning
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

“Things often go swimmingly for the first week. Try six weeks. By that time, having, as far as one can see, fallen back completely or even fallen lower than the point one began from, one will have discovered some truths about oneself. No man knows how bad he is till he has tried very hard to be good. A silly idea is current that good people do not know what temptation means. This is an obvious lie. Only those who try to resist temptation know how strong it is. After all, you find out the strength of the German army by fighting against it, not by giving in. You find out the strength of a wind by trying to walk against it, not by lying down.” source