Religious truth is relative, Divine Revelation is progressive

....The Revelation, of which Bahá'u'lláh is the source and center, abrogates none of the religions that have preceded it, nor does it attempt, in the slightest degree, to distort their features or to belittle their value. It disclaims any intention of dwarfing any of the Prophets of the past, or of whittling down the eternal verity of their teachings.

It can, in no wise, conflict with the spirit that animates their claims, nor does it seek to undermine the basis of any man's allegiance to their cause. Its declared, its primary purpose is to enable every adherent of these Faiths to obtain a fuller understanding of the religion with which he stands identified, and to acquire a clearer apprehension of its purpose. It is neither eclectic in the presentation of its truths, nor arrogant in the affirmation of its claims. Its teachings revolve around the fundamental principle that religious truth is not absolute but relative, that Divine Revelation is progressive, not final.

Unequivocally and without the least reservation it proclaims all established religions to be divine in origin, identical in their aims, complementary in their functions, continuous in their purpose, indispensable in their value to mankind.

Shoghi Effendi, The World Order of Bahá'u'lláh, p. 57