"O My Branches! In this Existent Being the greatest strength and the most perfect power is hidden and concealed; look towards It and gaze in the direction of Its union and not at Its seeming differences. This is the Testament of God that the Branches, Twigs, and Relations must each and every one look to the Greatest Branch.* Reflect upon that which is revealed in My Book, the Aqdas: “When the Ocean of My Presence hath disappeared and the Book of Origin is achieved to the end, turn your faces towards Him whom God hath purposed, Who hath branched from this Pre-Existent Root.” The aim of this blessed verse hath been the Greatest Branch. We have likewise elucidated the Command as a favor from before Us; and I am the Generous, the All-Giving!
Verily, God hath ordained the station of the Greater Branch after the station of the former. Verily, He is the Ordainer, the Wise. We have surely chosen the Greater after the Greatest as a Command from the All-Knowing, the Omniscient!"
"Greatest Branch" being translated from "Ghusn i Azam", which was Baha'u'llah's title for Abdul Baha, and "Greater Branch" from "Ghusn i Akbar", which was Baha'u'llah's translation for Mirza Muhammad Ali.
See this by a couple of scholars translating Baha'u'llah:
In another tablet revealed in response to that controversy, Bahá’u’lláh rejects Muḥammad-‘Alí's claim to revelation, states that he "
is but one of My servants... Should he for a moment pass out from under the shadow of the Cause, he surely shall be brought to naught," and mentions the extraordinary station of one of His sons.[^59] Later, in His Book of the Covenant, He would state that this high station belongs to ‘Abdu’l-Bahá, that Muḥammad-‘Alí is subordinate to him, and that all must turn toward ‘Abdu’l-Bahá.[^60] [59]: Bahá’u’lláh's tablet can be found in Ishraq Khavari, Ma'idiy-i-Asmani 1:56-68. See also Ma'idiy-i-Asmani 4:361, and Shoghi Effendi, God Passes By, 251. [60]: See Bahá’u’lláh, Tablets, 221.
Nader Saiedi, "Logos and Civilization"
Revealing more about the circumstances:
In this Tablet He declares His own station and states that as long as His sons observe the commandments of God, persevere in edifying their souls, testify to what has been revealed by God, believe in 'Him Whom God shall make manifest', do not create divisions in His Cause and do not deviate from His revealed laws, they can be considered as the leaves and branches of His tree of holiness and members of His family. Through them will the light of God be diffused and the signs of His bounty be made manifest.
Mírzá Muḥammad-‘Alí did not live up to these standards. Apart from his shameful claim of equality with Bahá’u’lláh, he became a source of sedition in the community, inflicted severe injuries upon the Cause of God and, after Bahá’u’lláh's ascension, broke His Covenant and rose up to extinguish the light of His Faith.
In the Tablet of Khalil Bahá’u’lláh alludes to ‘Abdu’l-Bahá in terms which immensely exalt Him above His other sons. He refers to Him as One 'from Whose tongue God will cause the signs of His power to stream forth' and as the One Whom 'God hath specially chosen for His Cause'.[^176] [176]: Bahá’u’lláh, quoted in Shoghi Effendi, God Passes By, p. 242.
In another Tablet[^177] revealed at this time when a few believers had been influenced by Mírzá Muḥammad-‘Alí's claim, Bahá’u’lláh asserts that when Muḥammad-‘Alí was a child of tender years He conferred upon him the power of utterance, so that people might witness His might and glory. He grieves in this Tablet at the state of some of His foolish followers who have thought to recognize a partner with Him in revelation and who have made great mischief in the land. He expresses astonishment at the behaviour of some who have attained His presence and witnessed the outpouring of His Revelation and yet have spread such shameful rumours among the believers. Referring to Muḥammad-‘Alí in this Tablet, He further states: [177]: Ma'idiy-i-Asmani vol. 8, p. 40.
He verily, is but one of My servants... Should he for a moment pass out from under the shadow of the Cause, he surely shall be brought to naught.[^178] [178]: Bahá’u’lláh, quoted in Shoghi Effendi, God Passes By, p. 251.
In this Tablet Bahá’u’lláh further confirms that all beings are created through a word from Him and that no one can claim equality, likeness or partnership with Him. He and He alone is the possessor of the [pg 171] Most Great Infallibility which is the prerogative of every Manifestation of God.
Concerning the three believers in Qazvín who were misled by Muḥammad-‘Alí's claim, Bahá’u’lláh invited Ḥájí Ḥasan and his brother to come to Adrianople. Here they attained His presence and fully recognized their folly.
Adib Taherzadeh, "The Child of the Covenant"
This is from the Will and Testament of Abdu'l-Baha:
Consider, how foolish are the people! They that have been in His (Bahá'u'lláh's) Presence and beheld His Countenance, have nevertheless noised abroad such idle talk, until, exalted be His explicit words, He said: -- "
Should he for a moment pass out from under the shadow of the Cause, he surely shall be brought to naught." Reflect! What stress He layeth upon one moment's deviation: that is, were he to incline a hair's breadth to the right or to the left, his deviation would be clearly established and his utter nothingness made manifest. And now ye are witnessing how the wrath of God hath from all sides afflicted him and how day by day he is speeding towards destruction. Ere long will ye behold him and his associates, outwardly and inwardly, condemned to utter ruin.
What deviation can be greater than breaking the Covenant of God! What deviation can be greater than interpolating and falsifying the words and verses of the Sacred Text, even as testified and declared by Mirza Badi'u'llah! What deviation can be greater than calumniating the Center of the Covenant himself! What deviation can be more glaring than spreading broadcast false and foolish reports touching the Temple of God's Testament! What deviation can be more grievous than decreeing the death of the Center of the Covenant, supported by the holy verse: -- "He that layeth a claim ere the passing of a thousand years...," whilst he (Muhammad Ali) without shame in the days of the Blessed Beauty had advanced such a claim as this and been confuted by Him in the aforementioned manner, the text of his claim being still extant in his own handwriting and bearing his own seal.
(Abdu'l-Baha, The Will and Testament, p. 6)
I provided the two scholars above translating the Tablet of Baha'u'llah, so you know that Abdu'l-Baha was not making this up for his own purposes.
Mirza Muhammad Ali had broken the Covenant of Baha'u'llah and thus had invalidated his succession to Abdu'l-Baha. What ruin such a person would have wreaked upon the Baha'i Cause!