The fundamental principle enunciated by Bahá’u’lláh …

The fundamental principle enunciated by Bahá’u’lláh … is that religious truth is not absolute but relative, that Divine Revelation is a continuous and progressive process, that all the great religions of the world are divine in origin, that their basic principles are in complete harmony, that their aims and purposes are one and the same, that their teachings are but facets of one truth, that their functions are complementary, that they differ only in the nonessential aspects of their doctrines, and that their missions represent successive stages in the spiritual evolution of human society…. …
His mission is to proclaim that the ages of the infancy and of the childhood of the human race are past, that the convulsions associated with the present stage of its adolescence are slowly and painfully preparing it to attain the stage of manhood, and are heralding the approach of that Age of Ages when swords will be beaten into plowshares, when the Kingdom promised by Jesus Christ will have been established, and the peace of the planet definitely and permanently ensured. Nor does Bahá’u’lláh claim finality for His own Revelation, but rather stipulates that a fuller measure of the truth He has been commissioned by the Almighty to vouchsafe to humanity, at so critical a juncture in its fortunes, must needs be disclosed at future stages in the constant and limitless evolution of mankind.

Shoghi Effendi: The Promised Day Is Come