that's because a simple reason: These principles and laws, these firmly-established and mighty systems, have proceeded from one Source and are the rays of one Light. That they differ one from another is to be attributed to the varying requirements of the ages in which they were promulgated. Bahá'u'lláh.
Thus the principle of the unity of religion means that all of the great religious Founders--the Manifestations--have come from God, and that all of the religious systems established by Them are part of a single divine plan directed by God.
In reality, there is only one religion, the religion of God. This one religion is continually evolving, and each particular religious system represents a stage in the evolution of the whole. The Bahá'í Faith represents the current stage in the evolution of religion. To emphasize the idea that all of the teachings and actions of the Manifestation are directed by God and do not originate from natural, human sources, Bahá'u'lláh used the term "revelation" to describe the phenomenon that occurs each time a Manifestation appears. In particular, the writings of the Manifestation represent the infallible Word of God. Because these writings remain long after the earthly life of the Manifestation is finished, they constitute an especially important part of the phenomenon of revelation. So much is this so, that the term "revelation" is sometimes used in a restricted sense to refer to the writings and words of the Manifestation.
These principles and laws, these firmly-established and mighty systems, have proceeded from one Source and are the rays of one Light. That they differ one from another is to be attributed to the varying requirements of the ages in which they were promulgated.
Bahá'u'lláh.
Thus the principle of the unity of religion means that all of the great religious Founders--the Manifestations--have come from God, and that all of the religious systems established by Them are part of a single divine plan directed by God.
In reality, there is only one religion, the religion of God. This one religion is continually evolving, and each particular religious system represents a stage in the evolution of the whole. The Bahá'í Faith represents the current stage in the evolution of religion. To emphasize the idea that all of the teachings and actions of the Manifestation are directed by God and do not originate from natural, human sources, Bahá'u'lláh used the term "revelation" to describe the phenomenon that occurs each time a Manifestation appears. In particular, the writings of the Manifestation represent the infallible Word of God. Because these writings remain long after the earthly life of the Manifestation is finished, they constitute an especially important part of the phenomenon of revelation. So much is this so, that the term "revelation" is sometimes used in a restricted sense to refer to the writings and words of the Manifestation.
(Source: http://info.bahai.org/article-1-4-0-4.html)
feel free to ask ;)