The basic unity of all religions.
Blessed souls—whether Moses, Jesus, Zoroaster, Krishna, Buddha, Confucius or Muḥammad—were the cause of the illumination of the world of humanity. How can we deny such irrefutable proof? How can we be blind to such light? How can we dispute the validity of Christ? This is injustice. This is a denial of reality. Man must be just. We must set aside bias and prejudice. We must abandon the imitations of ancestors and forefathers. We ourselves must investigate reality and be fair in judgment.
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. ... 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! Wherefore, should one of these Manifestations of Holiness proclaim saying: "I am the return of all the Prophets," He, verily, speaketh the truth. ...
The other station is the station of distinction, and pertaineth to the world of creation, and to the limitations thereof. 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.
Bahá’u’lláh, Gleanings from the Writings of Bahá’u’lláh, p. 50.
"The Blessed Beauty (Baha'u'llah) said: "All are the fruits of one tree and the leaves of one branch." He likened the world of existence to one tree and all the souls to leaves, blossoms and fruits. Therefore all the branches, leaves, blossoms and fruits must be in the utmost of freshness, and the bringing about of this delicacy and sweetness depends upon union and fellowship. Therefore they must assist each other with all their power and seek everlasting life. Thus the friends of God must manifest the mercy of the Compassionate Lord in the world of existence and must show forth the bounty of the visible and invisible King. They must purify their sight, and look upon mankind as the leaves, blossoms and fruits of the tree of creation, and must always be thinking of doing good to someone, of love, consideration, affection and assistance to somebody. They must see no enemy and count no one as an ill wisher. They must consider every one on the earth as a friend; regard the stranger as an intimate, and the alien as a companion. They must not be bound by any tie, nay, rather, they should be free from every bond. In this day the one who is favored in the threshold of grandeur is the one who offers the cup of faithfulness and bestows the pearl of gift to the enemies, even to the fallen oppressor, lends a helping hand, and considers every bitter foe as an affectionate friend.
These are the commands of the Blessed Beauty, these are the counsels of the Greatest Name. O ye dear friends! The world is engaged in war and struggle, and mankind is in the utmost conflict and danger. The darkness of unfaithfulness has enshrouded the earth and the illumination of faithfulness has become concealed. All nations and tribes of the world have sharpened their claws and are warring and fighting with each other. The edifice of man is shattered. Thousands of families are wandering disconsolate. Thousands of souls are besmeared with dust and blood in the arena of battle and struggle every year, and the tent of happiness and life is overthrown. The prominent men become commanders and boast of bloodshed, and glory in destruction. One says: "I have severed with my sword the necks of a nation," and one: "I have leveled a kingdom to the dust"; and another: "I have overthrown the foundation of a government." This is the pivot around which the pride and glory of mankind are revolving. In all regions friendship and uprightness are denounced and reconciliation and regard for truth are despised. The herald of peace, reformation, love and reconciliation is the Religion of the Blessed Beauty which has pitched its tent on the apex of the world and proclaimed its summons to the people."
(Abdu'l-Baha, Baha'i World Faith - Abdu'l-Baha Section, p. 215)
This was written in the late 19th, early 20th century! And man is still mired in war and self destruction in the 21st century!
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. If any man, therefore, should fail to comprehend this truth, and should consequently indulge in vain and unseemly language, no one whose sight is keen and whose understanding is enlightened would ever allow such idle talk to cause him to waver in his belief.
The measure of the revelation of the Prophets of God in this world, however, must differ. Each and every one of them hath been the Bearer of a distinct Message, and hath been commissioned to reveal Himself through specific acts. It is for this reason that they appear to vary in their greatness. Their Revelation may be likened unto the light of the moon that sheddeth its radiance upon the earth. Though every time it appeareth, it revealeth a fresh measure of its brightness, yet its inherent splendor can never diminish, nor can its light suffer extinction.
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. 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.
from Gleanings by Bahá'u'lláh