Bhagavad Gita Verse 6, Chapter 17

karshayantaha shareerastham bhootagraamamachetasaha |
maam chaivaantahashareerastham taanviddhyaasuranishchayaan || 6 ||
 
The collection of elements situated in the body, as well as me who dwells in the body, are weakened by ignorant people. Know them to be of devilish resolves.
 
karshayantaha : weakened
shareerastham : situated in body
bhootagraamam : collection of elements
achetasaha : ignorant people
maam : me
cha : and
eva : also
antahashareerastham : dwells in the body
taan : them
viddhi : know
aasuranishchayaan : devilish resolves
 
Let's say it is the first day of the new year, and we make a resolution to lose some weight. The very next day, we join a gym for the very first time, and spend one hour running on the treadmill. This will not be a good idea. Instead of helping our body, we may harm it. Weight loss, like any other system, requires a combination of knowledge and action. Unless we understand the techniques involved in weight loss, we should avoid starting an exercise regimen.
 
A similar state is found in some people who practice terrible austerities in the name of worship. Shri Krishna explains their fate here. He says that those who torture the body and mind in the name of austerity are doing nothing but weakening the elements, meaning the organs, in their body. Nowhere in the scriptures does it ask us to harm our body. He goes one step further by asserting that such people are also hurting Ishvara himself. As we saw in the ninth chapter, Ishvara dwells in all of us as the saakshi, the witness of our thoughts and actions.
 
Shri Krishna calls such people achetesaaha which means ignorant, or even unconscious. They chose to ignore Ishvara's teaching in the form of scriptures. They do not have faith in any ideal except their own preservation. They are driven only by egoism and desire. It is no wonder that their plans, their resolves, will be aasuric or devilish, as explained in the prior chapter. So the warning to us is clear. As we are conducting the analysis of our faith, we should in no way emulate these people. We should stay away from them.