These religions are not the only true religions that have appeared in the world, but are the only ones still existing.

"The number nine which in itself is the number of perfection is considered by the Bahá'ís as sacred because it is symbolic of the perfection of the Bahá'í Revelation which constitutes the ninth in the line of existing religions, the latest and fullest Revelation which mankind has ever known. The eighth is the religion of the Bab, and the remaining seven are: Hinduism, Buddhism, Zoroastrianism, Judaism, Christianity, Islam, and the religion of the Sabeans. These religions are not the only true religions that have appeared in the world, but are the only ones still existing. There have always been divine Prophets and Messengers, to many of whom the Qur'an refers. But the only ones existing are those mentioned above."

(From a letter written on behalf of the Guardian to an individual believer, July 28, 1936: Bahá'í News, No. 105, February 1937, p. 2)

http://bahai-library.com/hornby_lights_guidance_2.html&chapter=4#n1373