By their fruit you will recognize them
…every good tree bears good fruit, but a bad tree bears bad fruit. A good tree cannot bear bad fruit, and a bad tree cannot bear good fruit. …Thus, by their fruit you will recognize them.
Words of Jesus as recorded in Matthew 7:15-20
The proofs applicable to one Prophet are equally applicable to another.
‘Abdu’l-Bahá The Promulgation of Universal Peace
If thou deniest Me, by what proof canst thou vindicate the truth of that which thou dost possess?
Bahá’u’lláh Lawh-I-Burhán (Tablet of the Proof)
The proof of the sun is the light thereof, which shineth and envelopeth all things.
Bahá’u’lláh, Kitáb-i-Íqán
Are miracles important?
…in the sight of the Manifestations of God these marvels and miracles are of no importance, so much so that They do not even wish them to be mentioned. For even if these miracles were considered the greatest of proofs, they would constitute a clear evidence only for those who were present when they took place, not for those who were absent.
‘Abdu’l‑Bahá, Some Answered Questions.
Proofs must be in connection with the subject which is to be proven
…know ye – May God illumine your insight, increase the purity of your conscience, enlighten your hearts and minds through His Manifest Light! – that the proofs and arguments must needs be in connection with the subject which is to be proven and demonstrated; otherwise they cannot be accounted as proofs, even though they might be very wonderful and astonishing. For instance, if a certain person claims to be a physician and seeks to prove the validity of his claim by flying in the air, the truth of his claim would not, of necessity, be proven, even if he did so. Flight in the air is not a characteristic endowment of a physician, and there is no connection between the medical profession and flight, even though soaring in the air is extremely amazing. But healing sicknesses, removing diseases, physical diagnosis and treatment are characteristics of medical science and confirmatory evidences of the truth of a physician’s claim. Again, if a man claims to be an engineer, his proof cannot be in quickening the dead. If he claims to be a writer, his evidence is not in the fact that he causes stones to speak; or if he claims to be an orator, his proof is not in cleaving the sea asunder, etc. Such performances cannot prove their respective statements, validate their claims, nor be considered as proofs and arguments. There is no connection whatever between these practices and claims; as is clearly and decisively evident to such as reflect upon the meaning of proof and argument.
Abu’l-Fadl, Mirza, The Bahá’í Proofs, Bahá’í Publishing Trust, Wilmette, Illinois, 1983, pp.189-190
The first and foremost testimony…
The first and foremost testimony establishing His truth is His own Self. Next to this testimony is His Revelation. For whoso faileth to recognize either the one or the other He hath established the words He hath revealed as proof of His reality and truth. This is, verily, an evidence of His tender mercy unto men. He hath endowed every soul with the capacity to recognize the signs of God. How could He, otherwise, have fulfilled His testimony unto men, if ye be of them that ponder His Cause in their hearts. He will never deal unjustly with any one, neither will He task a soul beyond its power. He, verily, is the Compassionate, the All-Merciful.
Gleanings from the Writings of Bahá’u’lláh
Transformation
And yet, is not the object of every Revelation to effect a transformation in the whole character of mankind, a transformation that shall manifest itself both outwardly and inwardly, that shall affect both its inner life and external conditions? For if the character of mankind be not changed, the futility of God’s universal Manifestations would be apparent.
Bahá’u’lláh, The Kitab-i-Iqan
The recurring proof
The recurring proof of the existence of God, therefore, is that from time immemorial He has repeatedly manifested Himself. In the larger sense, as Bahá’u’lláh explains, the vast epic of humanity’s religious history represents the fulfilment of the “Covenant”, the enduring promise by which the Creator of all things assures the race of the unfailing guidance essential to its spiritual and moral development, and calls on it to internalize and give expression to these values. One is free to dispute through historicist interpretations of the evidence the unique role of this or that Messenger of God, if that is one’s purpose, but such speculation is of no help in accounting for developments that have transformed thought and produced changes in human relationships critical to social evolution. At intervals so rare that the known instances can be counted on one’s fingers, the Manifestations of God have appeared, have each been explicit as to the authority of His teachings and have each exerted an influence on the advance of civilization incomparably beyond that of any other phenomenon in history.
(One Common Faith)