Every Bahá’í is at liberty, nay is urged, to freely express his opinion and his understanding of the Teachings, but all this is in a totally different category from that of a Bahá’í who opposes the clear Teachings of Bahá’u'lláh or who asserts his own opinion as an authoritative and correct interpretation of the teachings, and attacks or opposes the very Institutions which Bahá’u'lláh has created to protect His Covenant. When a person declares his acceptance of Bahá’u'lláh as a Manifestation of God he becomes a party to the Covenant and accepts the totality of His Revelation. If he then turns round and attacks Bahá’u'lláh or the Central Institution of the Faith he violates the Covenant. If this happens every effort is made to help that person to see the illogicality and error of his actions, but if he persists he must, in accordance with the instructions of Bahá’u'lláh Himself, be shunned as a Covenant-breaker.
Letter from the Universal House of Justice, dated March 23, 1975, Developing Distinctive Bahá’í Communities

A Covenant-breaker…is one who disobeys and turns away from the Center of the Covenant.
Written on behalf of Shoghi Effendi, The Light of Divine Guidance v II, p. 109

To maintain the unity and incorruptibility of the Faith, the Covenant of Bahá’u'lláh established a Centre of authority to which all are to turn. This Centre has been, successively, ‘Abdu’l-Bahá, Who is uniquely the Centre of the Covenant; Shoghi Effendi, the Guardian of the Faith; and the Universal House of Justice, which is the apex of its Administrative Order. A Bahá’í who turns against and defies this Centre breaks the Covenant and, if he is adamant in his disobedience, is expelled from the Faith as a Covenant-breaker.
Footnote in Messages of the Universal House of Justice 1963 to 1986, p. 16

…one of the greatest and most fundamental principles of the Cause of God is to shun and avoid entirely the Covenant-breakers, for they will utterly destroy the Cause of God, exterminate His Law and render of no account all efforts exerted in the past.
Abdu’l-Baha, Will and Testament of Abdu’l-Baha, paragraph 38

…it is incumbent upon the friends that are fast and firm in the Covenant and Testament to be ever wakeful lest after this wronged one is gone this alert and active worker of mischief [Mirza Muhammad-Ali] may cause disruption, privily sow the seeds of doubt and sedition and utterly root out the Cause of God. A thousand times shun his company. Take heed and be on your guard. Watch and examine; should anyone, openly or privily, have the least connection with him, cast him out from your midst, for he will surely cause disruption and mischief.

O ye beloved of the Lord! Strive with all your heart to shield the Cause of God from the onslaught of the insincere, for souls such as these cause the straight to become crooked and all benevolent efforts to produce contrary results.

Abdu’l-Baha, Will and Testament of Abdu’l-Baha, paragraphs 41-42

The seriousness of Covenant-breaking is that it strikes at the very center and foundation of the unity of mankind. If God were to allow the instrument to be divided and impaired, how then would His purpose be achieved?

. . . The believers are commanded to shun Covenant-breakers for the same reason as healthy people do not associate with a person suffering from a serious contagious illness. A contagiously sick person cannot catch health from a thousand healthy people, but, on the contrary, he can infect them with his illness. Therefore such a person is quarantined and only those few people qualified to attend him do so. . . . Then, if he proves adamant he is free to go his way, but the Bahá’ís must cease to have any association with him until such time as he repents when, of course, he can be accepted back into the community.
Universal House of Justice, March 23, 1975, Developing Distinctive Bahá’í Communities

These do not doubt the validity of the Covenant but selfish motives have dragged them to this condition. It is not that they do not know what they do — they are perfectly aware and still they exhibit opposition.
Abdu’l-Baha, Selections from the Writings of Abdu’l-Baha, p. 215

No proper picture of Shoghi Effendi’s life can be obtained without reference to the subject of Covenant-breaking. The principle of light and shadow, setting each other off, the one intensifying the other, is seen in nature and in history; the sun casts shadows; at the base of the lamp lies shadow; the brighter the light the darker the shadow; the evil in men calls to mind the good, and the greatness of the good underlines the evil. The entire life of the Guardian was plagued and blighted by the ambition, the folly, the jealousy and hatred of individuals who rose up against the Cause and against him as Head of the Cause and who thought they could either subvert the Faith entirely or discredit its Guardian and set themselves up as leaders of a rival faction and win the body of believers over to their own interpretation of the Teachings and the way in which they believed the Cause of God should be run. No one ever succeeded in doing these things, but a series of disaffected individuals never ceased to try. The ringleaders misled the fools, the excommunicated tried to pervert the faithful.
Ruhiyyih Khanum, The Priceless Pearl, p. 118

Ever since the opposition of the younger brother of Bahá’u'lláh, Mirza Yahya, the poison of Covenant-breaking, which is opposition to the Centre of the Covenant, entered the Faith and remained. It is difficult for those who have neither experienced what this disease is, nor devoted any consideration to the subject, to grasp the reality of the power for destruction it possesses. All the members of the family of Bahá’u'lláh grew up in the shadow of Covenant-breaking. The storms, separations, reconciliations, final sundering of ties, which are involved when a close, distinguished and often dear relative is dying spiritually of a spiritual disease, are inconceivable to one who has not experienced them. The weakness of the human heart, which so often attaches itself to an unworthy object, the weakness of the human mind, prone to conceit and self-assurance in personal opinions, involve people in a welter of emotions that blind their judgment and lead them far astray. In the East, where the sense of family to this day is still strongly clannish, its members cling to each other much more intensely than in the West. No matter what Yahya had done there was a lingering feeling in the family that, after all, some reason must be on his side, not all justification in a family matter was necessarily on Bahá’u'lláh’s side. One can readily see that if even the faintest trace of such an attitude existed amongst members of Bahá’u'lláh’s own family the children would not grow up to see Covenant-breaking in its true proportions. The flaw would be there, the most dangerous of all human doubts, that after all the Perfect One might not under all circumstances be perfect, but sometimes just a little prone to error in judging others. When this doubt enters, the germs are present in one’s own system, perhaps to lie dormant forever, perhaps to flare up into disease. It has always seemed to me that the division which took place in Bahá’u'lláh’s family after His ascension, and the successive disaffections two generations later of ‘Abdu’l-Bahá’s entire family from Shoghi Effendi, had begun in an attitude of mind that developed in the Baghdad days before Bahá’u'lláh had even declared His Mission. The root was back there, the poisonous fruit garnered eighty years later.

Faith and obedience are the most important factors in one’s relation to God, to His Manifestation, to the Head of the Faith. One must believe even if one does not see, and even if one does not believe one must obey. The Covenant-breaking inside the family of Bahá’u'lláh was like a vine, it entwined the tree and strangled it; wherever its tendrils reached out it plucked up what it wound itself about and destroyed that too. This is why so many of the minor relatives, the secretaries, the members of the community surrounding the Centre of the Cause, became involved in the periodic disaffections of various members of the family and every time one of these diseased members was lopped off, some blinded sympathizers went too.

It looks simple on paper. But when year after year a house is torn by heart-breaking emotions, shaken by scenes that leave one’s brain numb, one’s nerves decimated and one’s feelings in a turmoil, it is not simple, it is just plain hell. Before a patient lies on the operating table and the offending part is removed there is a long process of delay, of therapeutic effort to remedy the disease, of hope for recovery. So it is with Covenant-breaking; the taint is detected; warning, remonstrance, advice follow; it seems better; it breaks out again, worse than before; convulsive situations arise - repentance, forgiveness follow - and then all over again, the same thing, worse than before, recommences. With infinite variations this is what took place in the lifetimes of Bahá’u'lláh, ‘Abdu’l-Bahá and Shoghi Effendi.
Ruhiyyih Khanum, The Priceless Pearl, p. 121