The Covenant made by the Manifestation of God with His followers concerning His immediate successor is known as the Lesser Covenant. In the Kitáb-i-Aqdas and later in His Will and Testament known as the Kitáb-i-’Ahd, Bahá’u'lláh made such a covenant with His followers. Through these writings Bahá’u'lláh established a mighty and irrefutable covenant unprecedented in the annals of past religions. Never before has a Manifestation of God left behind an authoritative statement in which He has explicitly directed His people to turn to someone as His successor, or follow a defined system of administration for governing the religious affairs of the community.

The Gospels are silent on the question of successorship. Only a vague and inconclusive statement, ‘…thou art Peter, and upon this rock I will build my church’, has led some to consider Peter as Christ’s successor. Such a claim, which is not upheld by a clear and unequivocal declaration in the Gospels, has resulted in bitter conflicts throughout the checkered history of Christianity. As a result the religion founded by Christ has been divided into major sects from the early centuries, and these have multiplied in the course of time.

A similar situation arose in Islam. The story of Muhammad and the references He is reported to have made to Ali, His cousin and son-in-law, at Ghadir-i-Khumm is recounted by both the Shí’ah and Sunni sects of Islam, and each interprets it differently. The story is as follows:

Having completed the rites of pilgrimage to Mecca in the last year of His life, Muhammad, on His way back to Medina, ordered the large concourse of His followers to stop at a place known as Ghadir-i-Khumm. In that vast plain a number of saddles were stacked up, making an improvised pulpit from which Muhammad delivered an important address to the congregation. There, He is reported to have taken Ali by the hand and said, ‘Whoever considers Me as his Lord, then Ali is also his Lord.’

This is only a verbal statement, but the Shí’ah sect considers it to be authoritative and an indication that Ali is the lawful successor to the Prophet. But the majority of the Islamic people, the Sunnis, reject this view. The followers of Muhammad were divided into these two major sects almost immediately after His passing. The dire conflicts and fierce turmoils which have engulfed the nation of Islam ever since were, as testified by ‘Abdu’l-Bahá, the consequence of the bitter division which occurred after the death of the Prophet.

One of the distinguishing features of the Revelation of Bahá’u'lláh, then, is that its Author has established a mighty covenant with His followers concerning His successor, a covenant whose characteristics are delineated by Bahá’u'lláh Himself, a covenant written in His own hand, unequivocal in the provisions it has made for the future of His Cause, and acknowledged as an authentic document even by those who violated it. It is through this divinely ordained instrument alone that the unity of the Bahá’í community is preserved, the purity of its teachings safeguarded, and the incorruptibility of its institutions guaranteed. ‘This is a Day that shall not be followed by night,’ is Bahá’u'lláh’s own testimony in this regard.
Adib Taherzadeh, The Covenant of Bahá’u’lláh, p. 99

To direct and canalize these forces let loose by this Heaven-sent process, and to insure their harmonious and continuous operation after His ascension, an instrument divinely ordained, invested with indisputable authority, organically linked with the Author of the Revelation Himself, was clearly indispensable. That instrument Baha’u’llah had expressly provided through the institution of the Covenant, an institution which he had firmly established prior to His ascension. This same Covenant He had anticipated in His Kitab-i-Aqdas, had alluded to it as He bade His last farewell to the members of His family, who had been summoned to His bed-side, in the days immediately preceding His ascension, and had incorporated it in a special document which He designated as “the Book of My Covenant,” and which He entrusted, during His last illness, to His eldest son ‘Abdu’l-Baha.

Written entirely in His own hand … this unique and epoch- making Document, designated by Baha’u’llah as His “Most Great Tablet,” and alluded to by Him as the “Crimson Book” in His “Epistle to the Son of the Wolf,” can find no parallel in the Scriptures of any previous Dispensation, not excluding that of the Bab Himself. For nowhere in the books pertaining to any of the world’s religious systems, not even among the writings of the Author of the Babi Revelation, do we find any single document establishing a Covenant endowed with an authority comparable to the Covenant which Baha’u’llah had Himself instituted.
Shoghi Effendi, God Passes By , pp. 237-38

Baha’u’llah, the Revealer of God’s Word in this Day, the Source of Authority, the Fountainhead of Justice, the Creator of a new World Order, the Establisher of the Most Great Peace, the Inspirer and Founder of a world civilization, the Judge, the Lawgiver, the Unifier and Redeemer of all mankind, has proclaimed the advent of God’s Kingdom on earth, has formulated its laws and ordinances, enunciated its principles, and ordained its institutions. To direct and canalize the forces released by His Revelation He instituted His Covenant, whose power has preserved the integrity of His Faith, maintained its unity and stimulated its world-wide expansion throughout the successive ministries of ‘Abdu’l-Baha and Shoghi Effendi. It continues to fulfil its life-giving purpose through the agency of the Universal House of Justice whose fundamental object, as one of the twin successors of Baha’u’llah and ‘Abdu’l-Baha, is to ensure the continuity of that divinely-appointed authority which flows from the Source of the Faith, to safeguard the unity of its followers, and to maintain the integrity and flexibility of its teachings.
The Universal House of Justice, The Constitution of the Universal House of Justice