From the Writings of Baha’u’llah:
God’s purpose in sending His Prophets unto men is twofold. The first is to liberate the children of men from the darkness of ignorance, and guide them to the light of true understanding. The second is to ensure the peace and tranquillity of mankind, and provide all the means by which they can be established.
(Baha’u’llah: Gleanings, pp. 79-80)

The Prophets and Messengers of God have been sent down for the sole purpose of guiding mankind to the straight Path of Truth. The purpose underlying Their revelation hath been to educate all men, that they may, at the hour of death, ascend, in the utmost purity and sanctity and with absolute detachment, to the throne of the Most High.
(Baha’u’llah: Gleanings, pp. 156-157)

The door of the knowledge of the Ancient Being hath ever been, and will continue for ever to be, closed in the face of men. No man’s understanding shall ever gain access unto His holy court. As a token of His mercy, however, and as a proof of His loving-kindness, He hath manifested unto men the Day Stars of His divine guidance, the Symbols of His divine unity, and hath ordained the knowledge of these sanctified Beings to be identical with the knowledge of His own Self. Whoso recognizeth them hath recognized God. Whoso hearkeneth to their call, hath hearkened to the Voice of God, and whoso testifieth to the truth of their Revelation, hath testified to the truth of God Himself. Whoso turneth away from them, hath turned away from God, and whoso disbelieveth in them, hath disbelieved in God. Every one of them is the Way of God that connecteth this world with the realms above, and the Standard of His Truth unto every one in the kingdoms of earth and heaven. They are the Manifestations of God amidst men, the evidences of His Truth, and the signs of His glory.
(Baha’u’llah: Gleanings, pp. 49-50)

These Tabernacles of holiness, these primal Mirrors which reflect the light of unfading glory, are but expressions of Him Who is the Invisible of the Invisibles. By the revelation of these gems of divine virtue all the names and attributes of God, such as knowledge and power, sovereignty and dominion, mercy and wisdom, glory, bounty and grace, are made manifest.
(Baha’u’llah: The Kitab-i-Iqan, p. 103)

Be ye assured, moreover, that the works and acts of each and every one of these Manifestations of God, nay whatever pertaineth unto them, and whatsoever they may manifest in the future, are all ordained by God, and are a reflection of His Will and Purpose.
(Baha’u’llah: Gleanings, p. 59)

From the foregoing passages and allusions it hath been made indubitably clear that in the kingdoms of earth and heaven there must needs be manifested a Being, an Essence Who shall act as a Manifestation and Vehicle for the transmission of the grace of the Divinity Itself, the Sovereign Lord of all. Through the Teachings of this Day Star of Truth every man will advance and develop until he attaineth the station at which he can manifest all the potential forces with which his inmost true self hath been endowed. It is for this very purpose that in every age and dispensation the Prophets of God and His chosen Ones have appeared amongst men, and have evinced such power as is born of God and such might as only the Eternal can reveal.
(Baha’u’llah: Gleanings, pp. 67-68)

These Essences of Detachment, these resplendent Realities are the channels of God’s all-pervasive grace. Led by the light of unfailing guidance, and invested with supreme sovereignty, They are commissioned to use the inspiration of Their words, the effusions of Their infallible grace and the sanctifying breeze of Their Revelation for the cleansing of every longing heart and receptive spirit from the dross and dust of earthly cares and limitations. Then, and only then, will the Trust of God, latent in the reality of man, emerge, as resplendent as the rising Orb of Divine Revelation, from behind the veil of concealment, and implant the ensign of its revealed glory upon the summits of men’s hearts.
(Baha’u’llah: Gleanings, p. 67)

These Suns of Truth are the universal Manifestations of God in the worlds of His attributes and names, even as the visible sun that assisteth, as decreed by God, the true One, the Adored, in the development of all earthly things, such as the trees, the fruits, and colours thereof, the minerals of the earth, and all that may be witnessed in the world of creation, so do the divine Luminaries, by their loving care and educative influence, cause the trees of divine unity, the fruits of His oneness, the leaves of detachment, the blossoms of knowledge and certitude, and the myrtles of wisdom and utterance, to exist and be made manifest. Thus it is that through the rise of these Luminaries of God the world is made new, the waters of everlasting life stream forth, the billows of loving-kindness surge, the clouds of grace are gathered, and the breeze of bounty bloweth upon all created things. It is the warmth that these Luminaries of God generate, and the undying fires they kindle, which cause the light of the love of God to burn fiercely in the heart of humanity. It is through the abundant grace of these Symbols of Detachment that the Spirit of life everlasting is breathed into the bodies of the dead.
(Baha’u’llah: The Kitab-i-Iqan, pp. 33-34)

The Prophets of God should be regarded as physicians whose task is to foster the well-being of the world and its peoples, that, through the spirit of oneness, they may heal the sickness of a divided humanity.
(Baha’u’llah: Gleanings, p. 80)

From the Utterances of `Abdu’l-Baha:
What is the purpose of Prophethood? Why has God sent the Prophets? It is self-evident that the Prophets are the Educators of men and the Teachers of the human race. They come to bestow universal education upon humanity, to give humanity training, to uplift the human race from the abyss of despair and desolation and to enable man to attain the apogee of advancement and glory. The people are in darkness; the Prophets bring them into the realm of light. They are in a state of utter imperfection; the Prophets imbue them with perfections. The purpose of the prophetic mission is none other than the education and guidance of the people.
(`Abdu’l-Baha: Promulgation of Universal Peace, p. 405)

When the holy, divine Manifestations or Prophets appear in the world, a cycle of radiance, an age of mercy dawns. Everything is renewed. Minds, hearts and all human forces are reformed, perfections are quickened, sciences, discoveries and investigations are stimulated afresh, and everything appertaining to the virtues of the human world is revitalized.
(`Abdu’l-Baha: Promulgation of Universal Peace, p. 278)

The holy Manifestations are the ideal Gardeners of human souls, the divine Cultivators of human hearts. The world of existence is but a jungle of disorder and confusion, a state of nature producing nothing but fruitless, useless trees. The ideal Gardeners train these wild, uncultivated human trees, cause them to become fruitful, water and cultivate them day by day so that they adorn the world of existence and continue to flourish in the utmost beauty.
(`Abdu’l-Baha: Promulgation of Universal Peace, p. 466)

The purpose of this is to show that the holy Manifestations of God, the divine Prophets, are the first Teachers of the human race. They are universal Educators, and the fundamental principles they have laid down are the causes and factors of the advancement of nations.
(`Abdu’l-Baha: Promulgation of Universal Peace, p. 86)

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. 508)