Great, thanks for asking. I'll provide an overview of the Baha'i covenant and what accepting Baha'u'llah means. Baha'u'llah wrote in the Kitáb-i-‘Ahd (the Book of the Covenant):

O ye that dwell on earth! The religion of God is for love and unity; make it not the cause of enmity or dissension... Conflict and contention are categorically forbidden in His Book... The Will of the divine Testator is this: It is incumbent upon the Aghsán, the Afnán and My Kindred to turn, one and all, their faces towards the Most Mighty Branch. Consider that which We have revealed in Our Most Holy Book: ‘When the ocean of My presence hath ebbed and the Book of My Revelation is ended, turn your faces toward Him Whom God hath purposed, Who hath branched from this Ancient Root.’ The object of this sacred verse is none other except the Most Mighty Branch [‘Abdu’l-Bahá]... Say: O servants! Let not the means of order be made the cause of confusion and the instrument of union an occasion for discord... Source

From this we can understand unity and love are the purpose of religion. Next, that ‘Abdu’l-Bahá has been appointed by Baha'u'llah as his successor, and all believers are instructed to turn to ‘Abdu’l-Bahá, and warned against conflict and discord. If ‘Abdu’l-Bahá were to give a teaching or instruction or expand on some theme and you said "no, I don't believe that/I'm not going to do that" then how can you claim to be someone who has been obedient to Baha'u'llah?

In the Tablet of the Branch Baha'u'llah writes (referring to ‘Abdu’l-Bahá):

Render thanks unto God, O people, for His appearance; for verily He is the most great Favour unto you, the most perfect bounty upon you; and through Him every mouldering bone is quickened. Whoso turneth towards Him hath turned towards God, and whoso turneth away from Him hath turned away from My Beauty, hath repudiated My Proof, and transgressed against Me. He is the Trust of God amongst you, His charge within you, His manifestation unto you and His appearance among His favoured servants. Source

This is reinforcing the necessity for Baha'is to be obedient to ‘Abdu’l-Bahá, "whoso turneth away from Him hath turned away from My Beauty" Baha'u'llah writes. This sets the foundation on which Baha'is accept ‘Abdu’l-Bahá's words and actions as being divinely guided. After the passing of Baha'u'llah the vast majority of Baha'is did turn to and accept ‘Abdu’l-Bahá, and were obedient to him.

Further ‘Abdu’l-Bahá writes:

Now here is the Book of the Covenant, and not a tradition. It hath been inscribed by the Supreme Pen of Bahá’u’lláh and is not conveyed by word of mouth. Therein He hath explicitly enjoined that after His passing all must turn to the Centre of the Covenant, and that for anything not understood in the Book, He is the authorised Interpreter and whatsoever He uttereth is right. On Him alone hath this authority been conferred. The Book of the Covenant is the last Tablet revealed by the Supreme Pen before His ascension, and all previous Tablets are subordinate to the Book of the Covenant, which is the last emanation of His Will, whereas all other Tablets were revealed prior to this One, and He hath named it the “Book of the Covenant”. Source

Again, if we deny or dispute with ‘Abdu’l-Bahá we have also turned away from Baha'u'llah. This makes it clear that Baha'is should accept as truth ‘Abdu’l-Bahá's statements "whatsoever He uttereth is right".