Those who cavil at what I say and do not follow it, regard them as deluded in all ways. Know them to be verily lost, as they lack discrimination.
To make any instruction effective and persuasive, its outcome and reward have to be pronounced. The effect will be greater when the consequence of not following it is also sternly set forth. This is how Krishna goes on to speak about the sad plight of those who disregard his sublime instructions.
Human mind is multi-pronged. Some will cavil at Krishna or any other spiritual instructor like him, disparaging whatever he says. But by this, they only lose something very great and beneficial to their life.
Krishna says, delusion overpowers their intelligence in all fronts, making the whole life wasteful, unproductive and even harmful to the society.
To remove selfishness, inborn to the human, and imbibe selflessness is not easy. Yajµa accomplishes this feat, enriching individual, familial and societal life alike.