This was no longer true even when the question was originally posted in 2018.
In 1965, the government passed a blasphemy law that recognized only these 6 religions. On the identity card (KTP), under religion (agama), one was required to list either one of these 6 religions or "-" (blank).
This requirement was effectively repealed by the Constitutional Court ruling of 7 Nov 2017 (PDF: Indonesian, English).
Library of Congress report (2017):
The Law had required that followers of religions other than the major faiths leave the line for religion blank on their national ID cards. The Court decision came as a result of the challenge to the Law filed by four people who follow indigenous religions, whose argument was that the Law violated the ideal of equality before the law. ...
Justice Saldi Isra said that the disputed articles of the Law on Population Administration had created trouble for followers of faiths other than the major six, including problems in obtaining e-ID and family registration cards, marriage registration, and access to services provided by the civil administrative offices. ... Saldi added that now followers of native faiths should, when filing for family registration and e-ID cards, indicate that they are native faith followers (penghayat kepercayaan) and that they do not need to add other details. ...
Indonesians not identified with one of the major religions faced limits in education and employment, as well as marriage registration. ...
the ruling would also apply to followers of Baha’i and Judaism, faiths that are not indigenous to Indonesia but are not among the six religions that are formally recognized.
(Note that in practice, theone's ability of one to actually get somea religion other than the old official 6six listed on one's ID card may be limited. The old law was in place for over 50 years while the new ruling is less than 10 years old. Those processingOfficials who process applications may still be rejecting applications of those bold enough to put down say Judaism or atheism under religion.)