From the Writings of Baha’u’llah:
The fundamental purpose animating the Faith of God and His Religion is to safeguard the interests and promote the unity of the human race, and to foster the spirit of love and fellowship amongst men.
(Baha’u’llah: Gleanings, p. 215)

The purpose of religion as revealed from the heaven of God’s holy Will is to establish unity and concord amongst the peoples of the world; make it not the cause of dissension and strife. The religion of God and His divine law are the most potent instruments and the surest of all means for the dawning of the light of unity amongst men. The progress of the world, the development of nations, the tranquillity of peoples, and the peace of all who dwell on earth are among the principles and ordinances of God. Religion bestoweth upon man the most precious of all gifts, offereth the cup of prosperity, imparteth eternal life, and showereth imperishable benefits upon mankind.
(Baha’u’llah: Tablets of Baha’u’llah, pp. 129-130)

From the Writings and Utterances of `Abdu’l-Baha:
The central purpose of the divine religions is the establishment of peace and unity among mankind.
(`Abdu’l-Baha: Promulgation of Universal Peace, p. 98)

Fifty years ago Baha’u’llah declared the necessity of peace among the nations and the reality of reconciliation between the religions of the world. He announced that the fundamental basis of all religion is one, that the essence of religion is human fellowship and that the differences in belief which exist are due to dogmatic interpretation and blind imitations which are at variance with the foundations established by the Prophets of God. He proclaimed that if the reality underlying religious teaching be investigated all religions would be unified, and the purpose of God, which is love and the blending of human hearts, would be accomplished. According to His teachings if religious belief proves to be the cause of discord and dissension, its absence would be preferable; for religion was intended to be the divine remedy and panacea for the ailments of humanity, the healing balm for the wounds of mankind.
(`Abdu’l-Baha: Promulgation of Universal Peace, p. 354)

The cause of this fellowship and unity lies in the fact that the divine law has two distinct aspects or functions: one the essential or fundamental, the other the material or accidental. The first aspect of the revealed religion of God is that which concerns the ethical development and spiritual progress of mankind, the awakening of potential human susceptibilities and the descent of divine bestowals. These ordinances are changeless, essential, eternal. The second function of the divine religion deals with material conditions, the laws of human intercourse and social regulation. These are subject to change and transformation in accordance with the time, place and conditions.
(`Abdu’l-Baha: Promulgation of Universal Peace, pp. 97-98)

The purpose of all the divine religions is the establishment of the bonds of love and fellowship among men, and the heavenly phenomena of the revealed Word of God are intended to be a source of knowledge and illumination to humanity. So long as man persists in his adherence to ancestral forms and imitation of obsolete ceremonials, denying higher revelations of the divine light in the world, strife and contention will destroy the purpose of religion and make love and fellowship impossible. Each of the holy Manifestations announced the glad tidings of His successor, and each One confirmed the message of His predecessor. Therefore, inasmuch as They were agreed and united in purpose and teaching, it is incumbent upon Their followers to be likewise unified in love and spiritual fellowship. In no other way will discord and alienation disappear and the oneness of the world of humanity be established.
(`Abdu’l-Baha: Promulgation of Universal Peace, pp. 339-340)

The divine purpose in religion is pure love and agreement. The Prophets of God manifested complete love for all. Each One announced the glad tidings of His successor, and each subsequent One confirmed the teachings and prophecies of the Prophet Who preceded Him. There was no disagreement or variance in the reality of Their teaching and mission. Discord has arisen among Their followers, who have lost sight of reality and hold fast to imitations. If imitations be done away with and the radiant shining reality dawn in the souls of men, love and unity must prevail. In this way humanity will be rescued from the strife and wars which have prevailed for thousands of years; dissensions will pass away and the illumination of unity dawn.
(`Abdu’l-Baha: Promulgation of Universal Peace, p. 234)

Our purpose is to show how true religion promotes the civilization and honor, the prosperity and prestige, the learning and advancement of a people once abject, enslaved and ignorant, and how, when it falls into the hands of religious leaders who are foolish and fanatical, it is diverted to the wrong ends, until this greatest of splendors turns into blackest night.
(`Abdu’l-Baha: Secret of Divine Civilization, p. 80)

Universal benefits derive from the grace of the Divine religions, for they lead their true followers to sincerity of intent, to high purpose, to purity and spotless honor, to surpassing kindness and compassion, to the keeping of their covenants when they have covenanted, to concern for the rights of others, to liberality, to justice in every aspect of life, to humanity and philanthropy, to valor and to unflagging efforts in the service of mankind. It is religion, to sum up, which produces all human virtues, and it is these virtues which are the bright candles of civilization.
(`Abdu’l-Baha: Secret of Divine Civilization, p. 98)

From Letters Written on Behalf of Shoghi Effendi:
The inestimable value of religion is that when a man is vitally connected with it, through a real and living belief in it and in the Prophet Who brought it, he receives a strength greater than his own which helps him to develop his good characteristics and overcome his bad ones. The whole purpose of religion is to change not only our thoughts but our acts; when we believe in God and His Prophet and His Teachings, we find we are growing, even though we perhaps thought ourselves incapable of growth and change!
(Shoghi Effendi: Lights of Guidance, p. 209)

…the fundamental purpose of all religions – including our own – is to bring man nearer to God, and to change his character, which is of the utmost importance.
(Shoghi Effendi: Lights of Guidance, p. 506)