This is explained as the Twofold aspect of Faith. The aspect that is abrogated, which is old Covernant and old the Laws (replaced with the new laws, new Covernant) and the aspect that is eternal.
"The Bearers of the Trust of God are made manifest unto the peoples of the earth as the Exponents of a new Cause and the Revealers of a new Message. Inasmuch as these Birds of the celestial Throne are all sent down from the heaven of the Will of God, and as they all arise to proclaim His irresistible Faith, they, therefore, are regarded as one soul and the same person. For they all drink from the one Cup of the love of God, and all partake of the fruit of the same Tree of Oneness.
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. Even as He hath revealed: “No distinction do We make between any of His Messengers.” For they, one and all, summon the people of the earth to acknowledge the unity of God, and herald unto them the Kawthar of an infinite grace and bounty."............"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. Even as He saith: “Some of the Apostles We have caused to excel the others. To some God hath spoken, some He hath raised and exalted. And to Jesus, Son of Mary, We gave manifest signs, and We strengthened Him with the Holy Spirit.” Baha'u'llah.
Regards Tony