Should he make his Christian wife wear hijab?
Question

                I read many fatwas regarding hijab and niqab, and that wife is commanded to obey her husband, so long as it does not involve sin, and that there is no differentiation between a Muslim wife and a non-Muslim wife. My wife is Christian so I ask if I can make her wear hijab and niqab or dress modestly when out or non-mahram men are present for her own modesty and protection?.
Praise be to Allah.
    The husband should tell his wife to wear hijab, even if she 
    is Christian or Jewish, because the sin of her being uncovered and showing 
    her adornment will be on him. The husband has the right to instruct his wife 
    to do what she thinks it is permissible and he thinks is obligatory, and she 
    has to obey him in that regard, as discussed previously in the answer to 
    question no. 97125. 

    For more information please see also the answer to question 
    no. 70177

    The Standing Committee for Issuing Fatwas was asked: 

    In some countries it is rare for Muslim women to wear hijab. 
    A man married a Muslim woman and she does not want to wear hijab. Should he 
    divorce her or what should he do? Another Muslim married a kitaabi (Jewish 
    or Christian) woman and she does not want to wear hijab either. What is the 
    ruling? 

    They replied: 

    The woman who refuses to cover herself in front of non-mahram 
    men is regarded as disobedient to her husband and going against the laws of 
    Allah. Her husband should advise her to wear proper shar‘i hijab, and if she 
    does not respond then he should divorce her, whether she is Muslim or Jewish 
    or Christian, so as to keep away from evil and protect the family from sin. 
    And Allah is the source of strength. May Allah send blessings and peace upon 
    our Prophet Muhammad and his Family and Companions.

    End quote from Fataawa al-Lajnah, 17/108 

    Note: 

    Hijab is obligatory for Christian women, as it says in 
    Didascalia Apostolorum (Teachings of the Apostles) which is regarded as 
    the secondary book on religious law for Orthodox Christians. On p. 72 it 
    says: 

    “Thou therefore that art a Christian, do not imitate such 
    women; but if thou wouldst be a faithful woman, please thy husband only. And 
    when thou walkest in the street, cover thy head with thy robe, that by 
    reason of thy veil thy great beauty may be hidden. And adorn not thy natural 
    face; but walk with downcast looks, being veiled.

    [i. 9] And take heed that thou bathe not in a bath with men. 
    For when there is a women's bath in the city or in the village, a believing 
    woman may not bathe in a bath with men. For if thou coverest thy face from 
    strange men with a veil of modesty, how then canst thou go in with strange 
    men to a bath?” 

    End quote from Didascalia Apostolorum (Teachings of the 
    Apostles), translated into Arabic by the Hegumen Marqus Dawood, Maktabat 
    al-Mahabbah edn. (English translation by R. Hugh Connolly, Oxford: Clarendon 
    Press, 1929). 

    So think about this command to observe hijab and this 
    prohibition on displaying oneself and showing adornments to men. How many 
    Christian women follow these teaching nowadays?! 

    And Allah knows best.