"Rather than offering up ten thousand prayers for remedy, it would be better simply to outlaw this one evil." (WND -I, 15)
This passage tells us that in order to attain prosperity and peace for our society, rather than debating various methods and programs, it's better to first of all put an end to the "one evil" of erroneous beliefs, the fundamental cause of all unhappiness. This also applies to our own lives as individuals.
For example, let's say there's a person who gets sick because she doesn't get enough sleep or otherwise take care of herself properly. That neglect of her health is the "one evil" of her life. Unless she remedies that, no amount of proper nourishment will restore her to health. In the same way, there is "one evil" that obstructs us from carrying out our human revolution and becoming happy.
Your "one evil" may be becoming careless and belittling yourself when things don't go as you'd hoped. Another "one evil" is blaming one's own failings and unhappiness on others, and being unable to work together in harmony with others for kosenrufu. The tendency to give up and try to escape every time a difficulty presents itself is another "one evil." There are many other sorts of these "one evils," depending on the person.
The beginning of the challenge of human revolution starts with identifying the "one evil" in your own life, deciding to eliminate it, earnestly chanting Nam-myoho-renge-kyo and challenging yourself to succeed.
Shin'ichi then told them that their time in the young women's division is a vital period in their lives, because its when they build an enduring foundation. He stressed that if they wished to attain happiness in this life, it's essential to train and polish themselves while young and eradicate the "one evil" in their own lives.
"Strike while the iron is hot!" as the saying goes. Youth who polish their lives will be able to forge the kind of character to overcome any of life's hardships.
- Daisaku Ikeda - NHR Vol. 24 Chap. 3