Bahá'ís believe in three core principles:
The Oneness of God, The Oneness of Religion and The Oneness of Humanity.
God is unknowable in His Essence:
Bahá'u'lláh, Gleanings from the Writings of Bahá'u'lláh, 2d rev. ed. , pp. 64-65
Bahá’u’lláh explains that God is unknowable in His Essence. This means that people should not make images of God in their minds, thinking of Him, for example, as a man. In general, that which has been created cannot understand its creator.
“How wondrous is the unity of the Living, the Ever-Abiding God--a unity which is exalted above all limitations, that transcendeth the comprehension of all created things.... How lofty hath been His incorruptible Essence, how completely independent of the knowledge of all created things, and how immensely exalted will it remain above the praise of all the inhabitants of the heavens and the earth!”
Bahá'u'lláh, Gleanings, pp. 261-62
God is the Creator of all things. He has made the heavens and earth, with its mountains and valleys, its deserts and seas, its rivers, its meadows and trees. God has created the animals and God has created the human being. The reason behind our creation, is love.:
“O Son of Man! I loved thy creation, hence I created thee. Wherefore, do thou love Me, that I may name thy name and fill thy soul with the spirit of life.”
Bahá’u’lláh -The Hidden Words, Arabic No. 4
So although God’s existence is far beyond our understanding, His love touches our lives and our beings ceaselessly. The way this love flows to us is through His Eternal Covenant. According to this Eternal Covenant, God never leaves us alone and without guidance. Whenever humanity moves away from Him and forgets His Teachings, a Manifestation of God appears and makes His Will and Purpose known to us.
The Oneness of Religion:
The Manifestation of God
The word “manifest” means to reveal, to bring forth something that was not known before. The Manifestations of God are those special Beings Who reveal to humanity the Word and the Will of God; thus when we listen to Them, we are responding to the Call of God.
Example:
There is an example from the physical world that helps us to understand the concept of “Manifestation” as taught by Bahá’u’lláh. In this world, the sun is the source of all warmth and light, without which life would not exist on the planet. Yet the sun itself does not descend to earth, and if we tried to approach it, we would be totally consumed.
But suppose we take a well-polished mirror and point it towards the sun. In it we will see the image of the sun, and the more perfectly polished the mirror, the more perfect the image will be.
While God is beyond our reach, these perfect Beings come to us from time to time, live among us, give us guidance, and fill us with the energy we need to progress, materially and spiritually.
“Whatever is in the heavens and whatever is on the earth is a direct evidence of the revelation within it of the attributes and names of God, inasmuch as within every atom are enshrined the signs that bear eloquent testimony to the revelation of that Most Great Light.... To a supreme degree is this true of man.... For in him are potentially revealed all the attributes and names of God to a degree that no other created being hath excelled or surpasssed.... And of all men, the most accomplished, the most distinguished, and the most excellent are the Manifestations of the Sun of Truth. Nay, all else besides these Manifestations, live by the operation of their Will, and move and have their being through the outpourings of their grace.”
Bahá'u'lláh, Gleanings, pp. 177-79.
Bahá’u’lláh is The Manifestation of God for this Age
Bahá'u'lláh explained, that the essential connection between the individual and God is maintained through the writings and words of each Manifestation. Theses Manifestations teach the same truth. The only difference between them is that each have appeared in a different time in the history of humanity in order to help people to be educated according to the needs of that time. Bahá’u’lláh’s Teachings are in perfect harmony with the Teachings of all previous religions, such as Christianity, and Islam, but they address the condition of humanity today.
“This is the Day in which God’s most excellent favors have been poured out upon men, the Day in which His most mighty grace hath been infused into all created things. It is incumbent upon all the peoples of the world to reconcile their differences, and, with perfect unity and peace, abide beneath the shadow of the Tree of His care and loving-kindness.”
Bahá'u'lláh, Gleanings, p6.