Bahá’í Sacred Writings, ‘Abdu’l-Bahá, #480
O thou maidservant of God! Thy letter was received, and its contents testified to thy firmness in the Covenant. Therefore, it is my hope that thou mayest be assisted under all conditions. In this day the most important of all things is to be firm and steadfast in the Covenant and the Testament, for Bahá’í unity can in no wise be preserved except through the Covenant. If it could be preserved by any other means, the Blessed Beauty would undoubtedly have decreed it.
In the Kitáb-i-Aqdas, unto which everyone must turn, and in the Kitáb-i-‘Ahd, which is the last Tablet revealed by the Blessed Beauty and recorded by the Supreme Pen, He addresseth everyone in clear and explicit terms, bidding first the Ag̱hṣán, then the Afnán and His kindred, and finally all other believers, to turn unto the Centre of the Covenant. There is a verse revealed in the Kitáb-i-Aqdas exhorting all to turn, after His Ascension, to “Him Who hath branched from this Ancient Root”. In the Kitáb-i-‘Ahd, He testifieth in unmistakable terms that the object of this verse is none other than the Centre of the Covenant. And in a specific Tablet, the authenticity of which is admitted by everyone, He, in unequivocal language, identifieth the Centre of Sedition by name, declaring that should he pass out from under the shadow of the Cause in the slightest degree, he would be cut off from the Holy Tree. How could anything be more explicit than this? Now one must either say that the Blessed Beauty erred and led the people astray, for He directed them to obey someone who ought not to have been obeyed, or else say that the least deviation from the Covenant and the Testament entaileth deprivation from the bounties of Him Who is the Luminary of the world. Of these two alternatives, one must be true; there is no third.
In sum, Bahá’í unity cannot be preserved save through the Covenant of God. In this day, the dynamic power in the body of the world is the Covenant; if the Covenant be neglected, what other power can move it?