From the Writings of Baha’u’llah:
I testify before God that each one of these Manifestations hath been sent down through the operation of the Divine Will and Purpose, that each hath been the bearer of a specific Message, that each hath been entrusted with a divinely-revealed Book and been commissioned to unravel the mysteries of a mighty Tablet. The measure of the Revelation with which every one of them hath been identified had been definitely fore-ordained.
(Baha’u’llah: Gleanings, p. 74)

In this respect, each Manifestation of God hath a distinct individuality, a definitely prescribed mission, a predestined revelation, and specially designated limitations. Each one of them is known by a different name, is characterized by a special attribute, fulfils a definite mission, and is entrusted with a particular Revelation.
(Baha’u’llah: Gleanings, p. 52)

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.
(Baha’u’llah: The Kitab-i-Iqan, pp. 103-104)

Know of a certainty that in every Dispensation the light of Divine Revelation hath been vouchsafed unto men in direct proportion to their spiritual capacity. Consider the sun. How feeble its rays the moment it appeareth above the horizon. How gradually its warmth and potency increase as it approacheth its zenith, enabling meanwhile all created things to adapt themselves to the growing intensity of its light. How steadily it declineth until it reacheth its setting point. Were it, all of a sudden, to manifest the energies latent within it, it would, no doubt, cause injury to all created things…. In like manner, if the Sun of Truth were suddenly to reveal, at the earliest stages of its manifestation, the full measure of the potencies which the providence of the Almighty hath bestowed upon it, the earth of human understanding would waste away and be consumed; for men’s hearts would neither sustain the intensity of its revelation, nor be able to mirror forth the radiance of its light.
(Baha’u’llah: Gleanings, pp. 87-88)

It is clear and evident, therefore, that any apparent variation in the intensity of their light is not inherent in the light itself, but should rather be attributed to the varying receptivity of an ever-changing world. Every Prophet Whom the Almighty and Peerless Creator hath purposed to send to the peoples of the earth hath been entrusted with a Message, and charged to act in a manner that would best meet the requirements of the age in which He appeared.
(Baha’u’llah: Gleanings, p. 79)

No man, however acute his perception, can ever hope to reach the heights which the wisdom and understanding of the Divine Physician have attained. Little wonder, then, if the treatment prescribed by the physician in this day should not be found to be identical with that which he prescribed before. How could it be otherwise when the ills affecting the sufferer necessitate at every stage of his sickness a special remedy? In like manner, every time the Prophets of God have illumined the world with the resplendent radiance of the Day Star of Divine knowledge, they have invariably summoned its peoples to embrace the light of God through such means as best befitted the exigencies of the age in which they appeared.
(Baha’u’llah: Gleanings, p. 80)

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, p. 22)

From the Utterances of `Abdu’l-Baha:
These holy Manifestations have been as the coming of springtime in the world. Although the springtime of this year is designated by another name according to the changing calendar, yet as regards its life and quickening it is the same as the springtime of last year. For each spring is the time of a new creation, the effects, bestowals, perfections and life-giving forces of which are the same as those of the former vernal seasons, although the names are many and various. This is 1912, last year was 1911 and so on, but in fundamental reality no difference is apparent. The sun is one, but the dawning points of the sun are numerous and changing. The ocean is one body of water, but different parts of it have particular designations – Atlantic, Pacific, Mediterranean, Antarctic, etc. If we consider the names, there is differentiation; but the water, the ocean itself, is one reality.
(`Abdu’l-Baha: Promulgation of Universal Peace, p. 151)

From the Writings of Shoghi Effendi:
The successive Founders of all past Religions Who, from time immemorial, have shed, with ever-increasing intensity, the splendor of one common Revelation at the various stages which have marked the advance of mankind towards maturity may thus, in a sense, be regarded as preliminary Manifestations, anticipating and paving the way for the advent of that Day of Days when the whole earth will have fructified and the tree of humanity will have yielded its destined fruit.
(Shoghi Effendi: World Order of Baha’u’llah, p. 166)