From the Writings of Baha’u’llah:
Know thou assuredly that the essence of all the Prophets of God is one and the same. Their unity is absolute. God, the Creator, saith: There is no distinction whatsoever among the Bearers of My Message. They all have but one purpose; their secret is the same secret. To prefer one in honor to another, to exalt certain ones above the rest, is in no wise to be permitted. Every true Prophet hath regarded His Message as fundamentally the same as the Revelation of every other Prophet gone before Him.
(Baha’u’llah: Gleanings, pp. 78-79)

Furthermore, it is evident to thee that the Bearers of the trust of God are made manifest unto the peoples of the earth as the Exponents of a new Cause and the Bearers of a new Message. Inasmuch as these Birds of the Celestial Throne are all sent down from the heaven of the Will of God, and as they all arise to proclaim His irresistible Faith, they therefore are regarded as one soul and the same person. For they all drink from the one Cup of the love of God, and all partake of the fruit of the same Tree of Oneness.
(Baha’u’llah: The Kitab-i-Iqan, p. 152)

Beware, O believers in the Unity of God, lest ye be tempted to make any distinction between any of the Manifestations of His Cause, or to discriminate against the signs that have accompanied and proclaimed their Revelation. This indeed is the true meaning of Divine Unity, if ye be of them that apprehend and believe this truth. 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. Whoso maketh the slightest possible difference between their persons, their words, their messages, their acts and manners, hath indeed disbelieved in God, hath repudiated His signs, and betrayed the Cause of His Messengers.
(Baha’u’llah: Gleanings, pp. 59-60)

Thus, viewed from the standpoint of their oneness and sublime detachment, the attributes of Godhead, Divinity, Supreme Singleness, and Inmost Essence, have been and are applicable to those Essences of being, inasmuch as they all abide on the throne of divine Revelation, and are established upon the seat of divine Concealment. Through their appearance the Revelation of God is made manifest, and by their countenance the Beauty of God is revealed.
(Baha’u’llah: The Kitab-i-Iqan, pp. 177-178)

These attributes of God are not and have never been vouchsafed specially unto certain Prophets, and withheld from others. Nay, all the Prophets of God, His well-favoured, His holy, and chosen Messengers, are, without exception, the bearers of His names, and the embodiments of His attributes. They only differ in the intensity of their revelation, and the comparative potency of their light. Even as He hath revealed: “Some of the Apostles We have caused to excel the others.”(1) It hath therefore become manifest and evident that within the tabernacles of these Prophets and chosen Ones of God the light of His infinite names and exalted attributes hath been reflected, even though the light of some of these attributes may or may not be outwardly revealed from these luminous Temples to the eyes of men. That a certain attribute of God hath not been outwardly manifested by these Essences of Detachment doth in no wise imply that they Who are the Daysprings of God’s attributes and the Treasuries of His holy names did not actually possess it. Therefore, these illuminated Souls, these beauteous Countenances have, each and every one of them, been endowed with all the attributes of God, such as sovereignty, dominion, and the like, even though to outward seeming they be shorn of all earthly majesty.
(Baha’u’llah: The Kitab-i-Iqan, pp. 103-104)

These Manifestations of God have each a twofold station. One is the station of pure abstraction and essential unity. In this respect, if thou callest them all by one name, and dost ascribe to them the same attributes, thou hast not erred from the truth. Even as He hath revealed: “No distinction do We make between any of His Messengers.” For they, one and all, summon the people of the earth to acknowledge the unity of God, and herald unto them the Kawthar of an infinite grace and bounty. They are all invested with the robe of prophethood, and are honored with the mantle of glory. Thus hath Muhammad, the Point of the Qur’an, revealed: “I am all the Prophets.” Likewise, He saith: “I am the first Adam, Noah, Moses, and Jesus.” Similar statements have been made by Imam Ali. Sayings such as these, which indicate the essential unity of those Exponents of Oneness, have also emanated from the Channels of God’s immortal utterance, and the Treasuries of the gems of Divine knowledge, and have been recorded in the Scriptures. These Countenances are the recipients of the Divine Command, and the Day Springs of His Revelation. This Revelation is exalted above the veils of plurality and the exigencies of number. Thus He saith: “Our Cause is but One.” Inasmuch as the Cause is one and the same, the Exponents thereof also must needs be one and the same. Likewise, the Imams of the Muhammadan Faith, those lamps of certitude, have said: “Muhammad is our first, Muhammad is our last, Muhammad our all.”
It is clear and evident to thee that all the Prophets are the Temples of the Cause of God, Who have appeared clothed in divers attire. If thou wilt observe with discriminating eyes, thou wilt behold Them all abiding in the same tabernacle, soaring in the same heaven, seated upon the same throne, uttering the same speech, and proclaiming the same Faith. Such is the unity of those Essences of Being, those Luminaries of infinite and immeasurable splendor!
(Baha’u’llah: Gleanings, pp. 50-52)

Every discerning observer will recognize that in the Dispensation of the Qur’an both the Book and the Cause of Jesus were confirmed. As to the matter of names, Muhammad, Himself, declared: “I am Jesus.” He recognized the truth of the signs, prophecies, and words of Jesus, and testified that they were all of God. In this sense, neither the person of Jesus nor His writings hath differed from that of Muhammad and of His holy Book, inasmuch as both have championed the Cause of God, uttered His praise, and revealed His commandments. Thus it is that Jesus, Himself, declared: “I go away and come again unto you.” Consider the sun. Were it to say now, “I am the sun of yesterday,” it would speak the truth. And should it, bearing the sequence of time in mind, claim to be other than that sun, it still would speak the truth. In like manner, if it be said that all the days are but one and the same, it is correct and true. And if it be said, with respect to their particular names and designations, that they differ, that again is true. For though they are the same, yet one doth recognize in each a separate designation, a specific attribute, a particular character. Conceive accordingly the distinction, variation, and unity characteristic of the various Manifestations of holiness, that thou mayest comprehend the allusions made by the Creator of all names and attributes to the mysteries of distinction and unity, and discover the answer to thy question as to why that everlasting Beauty should have, at sundry times, called Himself by different names and titles….
(Baha’u’llah: Gleanings, pp. 21-22)

O Jews! If ye be intent on crucifying once again Jesus, the Spirit of God, put Me to death, for He hath once more, in My person, been made manifest unto you. Deal with Me as ye wish, for I have vowed to lay down My life in the path of God. I will fear no one, though the powers of earth and heaven be leagued against Me. Followers of the Gospel! If ye cherish the desire to slay Muhammad, the Apostle of God, seize Me and put an end to My life, for I am He, and My Self is His Self. Do unto Me as ye like, for the deepest longing of Mine heart is to attain the presence of My Best-Beloved in His Kingdom of Glory. Such is the Divine decree, if ye know it. Followers of Muhammad! If it be your wish to riddle with your shafts the breast of Him Who hath caused His Book the Bayan to be sent down unto you, lay hands on Me and persecute Me, for I am His Well-Beloved, the revelation of His own Self, though My name be not His name. I have come in the shadows of the clouds of glory, and am invested by God with invincible sovereignty. He, verily, is the Truth, the Knower of things unseen. I, verily, anticipate from you the treatment ye have accorded unto Him that came before Me. To this all things, verily, witness, if ye be of those who hearken. O people of the Bayan! If ye have resolved to shed the blood of Him Whose coming the Bab hath proclaimed, Whose advent Muhammad hath prophesied, and Whose Revelation Jesus Christ Himself hath announced, behold Me standing, ready and defenseless, before you. Deal with Me after your own desires.
(Baha’u’llah: Gleanings, pp. 101-102)

From the Utterances of `Abdu’l-Baha:
The holy Manifestations Who have been the Sources or Founders of the various religious systems were united and agreed in purpose and teaching. Abraham, Moses, Zoroaster, Buddha, Jesus, Muhammad, the Bab and Baha’u’llah are one in spirit and reality.
(`Abdu’l-Baha: Promulgation of Universal Peace, p. 197)

All these holy, divine Manifestations are one. They have served one God, promulgated the same truth, founded the same institutions and reflected the same light. Their appearances have been successive and correlated; each One has announced and extolled the One Who was to follow, and all laid the foundation of reality. They summoned and invited the people to love and made the human world a mirror of the Word of God.
(`Abdu’l-Baha: Promulgation of Universal Peace, p. 151)

The divine Manifestations have been iconoclastic in Their teachings, uprooting error, destroying false religious beliefs and summoning mankind anew to the fundamental oneness of God. All of Them have, likewise, proclaimed the oneness of the world of humanity. The essential teaching of Moses was the law of Sinai, the Ten Commandments. Christ renewed and again revealed the commands of the one God and precepts of human action. In Muhammad, although the circle was wider, the intention of His teaching was likewise to uplift and unify humanity in the knowledge of the one God. In the Bab the circle was again very much enlarged, but the essential teaching was the same. The Books of Baha’u’llah number more than one hundred. Each one is an evident proof sufficient for mankind; each one from foundation to apex proclaims the essential unity of God and humanity, the love of God, the abolition of war and the divine standard of peace.
(`Abdu’l-Baha: Promulgation of Universal Peace, p. 154)

From the Writings of Shoghi Effendi:
Though the mission of the Prophets preceding Baha’u’llah may be viewed in that light, though the measure of Divine Revelation with which each has been entrusted must, as a result of this process of evolution, necessarily differ, their common origin, their essential unity, their identity of purpose, should at no time and under no circumstances be misapprehended or denied. That all the Messengers of God should be regarded as “abiding in the same Tabernacle, soaring in the same Heaven, seated upon the same Throne, uttering the same Speech, and proclaiming the same Faith” must, however much we may extol the measure of Divine Revelation vouchsafed to mankind at this crowning stage of its evolution, remain the unalterable foundation and central tenet of Baha’i belief. Any variations in the splendor which each of these Manifestations of the Light of God has shed upon the world should be ascribed not to any inherent superiority involved in the essential character of any one of them, but rather to the progressive capacity, the ever-increasing spiritual receptiveness, which mankind, in its progress towards maturity, has invariably manifested.
(Shoghi Effendi: World Order of Baha’u’llah, p. 166)

1. Qur’an 2:253.