Bhishma tells Yudhishthira that the most important duties are worshipping and honoring one's mother, father, and preceptor. [1] He says serving them regularly allows one to attain heaven and worlds of felicity. [2] Bhishma emphasizes that the preceptor deserves the highest reverence and should never be disregarded, as they impart immortal knowledge and life to disciples. [3]

1. Most Important Duty
(From: The Mahabharata, Santi Parva, Section CVIII)
Yudhishthira said:
• 'The path of duty is long. It has also, O Bharata, many branches. What,
however, according to thee, are those duties that most deserve to be practiced?
• What acts, according to thee, are the most important among all duties, by the
practice of which I may earn the highest merit both here and hereafter?'
Bhishma said:
• 'The worship of mother, father, and preceptor is most important according to
me. The man, who attends to that duty here, succeeds in acquiring great fame
and many regions of felicity.
• Worshipped with respect by thee, whatever they will command thee, be it
consistent with righteousness or inconsistent with it, should be done
unhesitatingly, O Yudhishthira! One should never do what they forbid.
Without doubt, that which they command should always be done.
• They are the three worlds. They are the three modes of life. They are the three
Vedas. They are the three sacred fires. The father is said to be the Garhapatya fire;
the mother, the Dakshina fire, and the preceptor is that fire upon which libations
are poured. These three fires are, of course, the most eminent. If thou attendest
with heedfulness to these three fires, thou wilt succeed in conquering the
three worlds.
• By serving the father with regularity, one may cross this world. By serving the
mother in the same way, one may attain to regions of felicity in the next. By
serving the preceptor with regularity one may obtain the region of Brahman.
• Behave properly towards these three, O Bharata, thou shalt then obtain great
fame in the three worlds, and blessed be thou, great will be thy merit and
reward.
• Never transgress them in any act. Never eat before they eat, nor eat anything
that is better than what they eat. Never impute any fault to them. One should
always serve them with humility. That is an act of high merit. By acting in that
way, O best of kings, thou mayst obtain fame, merit, honour, and regions of
felicity hereafter.
• He who honours these three is honoured in all the worlds. He, on the other hand,
who disregards these three, fails to obtain any merit from any of his acts. Such a
man, O scorcher of foes, acquires merit neither in this world nor in the next.

2. • He who always disregards these three seniors never obtains fame either here
or hereafter. Such a man never earns any good in the next world.
• All that I have given away in honour of those three has become a hundredfold
or a thousand-fold of its actual measure. It is in consequence of that merit that
even now, O Yudhishthira, the three worlds are clearly before my eyes.
• One Acharya is superior to ten Brahmanas learned in the Vedas. One Upadhyaya
is again superior to ten Acharyas. The father, again, is superior to ten
Upadhyayas. The mother again, is superior to ten fathers, or perhaps, the
whole world, in importance. There is no one that deserves such reverence as
the mother.
• In my opinion, however, the preceptor is worthy of greater reverence than the
father or even the mother. The father and the mother are authors of one's being.
The father and the mother, O Bharata, only create the body. The life, on the
other hand that one obtains from one’s preceptor is heavenly. That life is
subject to no decay and is immortal.
• The father and the mother, however much they may offend, should never be
slain. By not punishing a father and a mother, (even if they deserve punishment),
one does not incur sin. Indeed, such reverend persons, by enjoying impunity, do
not stain the king. The gods and the Rishis do not withhold their favours from
such persons as strive to cherish even their sinful fathers with reverence.
• He who favours a person by imparting to him true instruction, by
communicating the Vedas, and giving knowledge which is immortal, should be
regarded as both a father and a mother. The disciple, in grateful recognition of
what the instructor has done, should never do anything that would injure the
latter.
• They that do not reverence their preceptors after receiving instruction from
them by obeying them dutifully in thought and deed, incur the sin of killing a
foetus. There is no sinner in this world like them.
• Preceptors always show great affection for their disciples. The latter should,
therefore, show their preceptors commensurate reverence. He, therefore, that
wishes to earn that high merit which has existed from ancient days, should
worship and adore his preceptors and cheerfully share with them every object
of enjoyment.
• With him who pleases his father is pleased Prajapati himself. He who pleases his
mother gratifies the earth herself. He who pleases his preceptor gratifies Brahma

3. by his act. For this reason, the preceptor is worthy of greater reverence than
either the father or the mother.
• If preceptors are worshipped, the very Rishis, and the gods, together with the
Pitris, are all pleased. Therefore, the preceptor is worthy of the highest
reverence. The preceptor should never be disregarded in any manner by the
disciple. Neither the mother nor the father deserves such regard as the
preceptor.
• The father, the mother, and the preceptor, should never be insulted. No act of
theirs should be found fault with. The gods and the great Rishis are pleased with
him that behaves with reverence towards his preceptors.
• They that injure in thought and deed their preceptors, or fathers, or mothers,
incur the sin of killing a foetus. There is no sinner in the world equal to them.
• That son of the sire's loins and the mother's womb, who, being brought up by
them and when he comes to age, does not support them in his turn, incurs the
sin of killing a foetus. There is no sinner in the world like unto him.
• We have never heard that these four, viz., he who injures a friend, he who is
ungrateful, he who slays a woman, and he who slays a preceptor, ever succeed
in cleansing themselves.
• I have now told thee generally all that a person should do in this world. Besides
those duties that I have indicated, there is nothing productive of greater felicity.
Thinking of all duties, I have told thee their essence.’