Who is to say what
    is halaal and haraam ?
Question

                Assalamu a'laikum How can we make things haraam or halaal (such as smoking or avoiding tax
      or insurance) when Allah (SWT) Himself is the only One to say what is halaal and what is
      not?
Praise be to Allah.Allaah, may He be exalted, says (interpretation of the meanings):
The command (or the judgement) is for none but Allaah. He has
    commanded that you worship none but Him [Yoosuf 12:40]
And say not concerning that which your tongues put forth falsely:
    This is lawful and this is forbidden., so as to invent lies against Allaah.
    Verily, those who invent lies against Allaah will never prosper. [al-Nahl 16:116]
Say [O Muhammad, to the polytheists]: Tell me, what
    provision Allaah has sent down to you! And you have made of it lawful and unlawful.
    Say [O Muhammad]: Has Allaah permitted you (to do so) or do you invent a lie against
    Allaah? [Yoonus 10:59]It is only for Allaah to say what is halaal and
    what is haraam, because He is the Law-giver, and there is no law-giver besides Him. When
    we say that a given thing is haraam, or halaal, we refer to the evidence in the Book of
    Allaah and the Sunnah of His Messenger 
 (peace and blessings of Allaah be upon him).
    Whatever we find is prohibited therein, we take as haraam, and whatever we find is
    permitted, we take as halaal. We refer to the sayings of the Sahaabah (Companions of the
    Prophet 
 (peace and blessings of Allaah be upon him) and the Taabieen (the
    generation after the Sahaabah)  may Allaah be pleased with them  and the
    commentaries of the Salaf (first generations of Islam) on the texts of the Quraan
    and Sunnah, and we refer to the sayings and ijtihaad of the scholars.
It is not permissible for us to allow or prohibit however we wish. When
    new issues come up, if we do not find a text in the Quraan or Sunnah, or in the
    sayings of the scholars of the salaf or the scholars of Ahl al-Sunnah
    wal-Jamaaah who came before us, then we have to refer to trustworthy scholars
    and people of understanding, as Allaah commands us (interpretation of the meaning): 
    so ask of those who know the Scripture [al-Nahl 16:43]. These are the
    scholars, who can do ijtihaad, examine the matter and make analogies (qiyaas) with the
    existing texts of Islam, taking into consideration the principles of necessity, and taking
    note of what is harmful to the interests of shareeah and what is beneficial, relying
    on the basic general principles of shareeah, such as the aayah (interpretation of
    the meaning),  he allows them as lawful al-tayyibaat [(i.e., all good and
    lawful things as regards things, deeds, beliefs, persons, foods, etc.]
    [al-Araaf 7:157] and the hadeeth, There should be no harming nor
    reciprocating harm (reported by Ibn Maajah, 2331), and
    avoiding following whims and desires. Every evil thing that is proven to be harmful is
    haraam, and every good and beneficial thing is halaal. If it is not known that it is
    either harmful or beneficial, then the general rule is that it is permissible (mubaah).
    And Allaah knows best.