According to articles 4, 5 and 6 define the ways to obtain Bahrain citizenship:
Bahrainis by descent:
4. Anyone shall be regarded a Bahraini national, if:
(A) Was born in Bahrain after the effective date of this act and his father was a
Bahraini at the time of birth.
(B) Born outside Bahrain, after the effective date of this Act, and his father was a
Bahraini national at the time of birth provided that this father or the
grandfather was born in Bahrain.
(C) Born in Bahrain or abroad, after the effective date of this Act, and his
mother, at the time of birth was a Bahraini national provided that father was
unknown, without nationality or fatherhood was not substantiated.
Bahrainis by birth
5. A person shall be regarded Bahraini by birth if:
(A) Born in Bahrain, after the effective date of this Act, and his father was also
born in Bahrain and has made Bahrain his permanent residence, at the time of
birth of that person, provided, however, that this person is not holding another
nationality.
(B) Born in Bahrain, after the effective date of this Act, to unknown parents.
Illegal child shall be deemed to have been born in Bahrain, unless otherwise
has been proved.
Bahrainis by naturalization:
6.Ìý
(1) The Bahraini Citizenship may be granted, by order from His Majesty the
   Governor, to any alien of full legal capacity, if requested by the alien who should
   meet the following requirements:
     (A) He has made Bahrain, his usual place of residence legally for at least twenty
        five consecutive years or fifteen years consecutively for Arab nationals,
        provided, however, that this period shall commences after the effective date
        of this act.
     (B) Shall be of good conduct
     (C) Must be conversant in Arabic.
     (D) Shall have a real estate in Bahrain registered in his name at the Land Registry
        Office of the Government of Bahrain.
(2) Notwithstanding, the above Bahraini citizenship may be granted to anyone, by
  order of His Majesty the Governor. Bahraini Citizenship may also be granted, by Order
  of His Majesty the Governor, to any Arab person, upon his request, if that person has
  rendered Bahrain great services.
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(3) An alien who has obtained the citizenship of Bahrain by virtues of this article, shall
  not be entitled to voting rights, representation, nomination or appointment in local
  councils (except clubs and private associations), except after the lapse of ten years
  from date of acquiring the citizenship. This provision is applicable to persons, who
  have acquired the Bahraini nationality before the effective date of this Act and the ten
  years span, begins from the effective date of this Act.
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(4) If a person is granted the citizenship of Bahrain by virtues of this article, his wife
  and minor children, shall be regarded Bahraini by naturalization, starting from the
  date of granting the nationality
Look for an abstract of the act