He wants to marry a Christian woman and he wants to convince her that it is forbidden to keep a dog in the house
Question

                Why is dog not permissible in the house ? I know it is mentioned in the hadith, I accept it because I am a muslim but how can I justify it to my future wife who is a christian, I need strong points ? Why ?.
Praise be to Allah.Firstly: 
Before answering this 
    question, you have to understand that your Prophet Muhammad  (peace and 
    blessings of Allaah be upon him) has encouraged you to make a good choice 
    when choosing a wife, the righteous wife whom the husband can trust with his 
    children, house and wealth. It was narrated from Abu Hurayrah (may Allaah be 
    pleased with him) that the Prophet  (peace and blessings of Allaah be 
    upon him) said: “Women may be married for four things: their wealth, their 
    lineage, their beauty and their religious commitment. Choose the one who is 
    religiously-committed, may your hands be rubbed with dust (i.e., may you 
    prosper).” Narrated by al-Bukhaari, 4802; Muslim, 1466. 
This advice from the 
    Prophet  (peace and blessings of Allaah be upon him) does not mean 
    choosing between a Muslim woman and a kitaabi woman (a Jewish or Christian 
    woman), rather it means choosing from among Muslim women, because if not 
    every Muslim woman is recommendable for marriage, then it is more likely 
    that it is inadvisable to marry a non-Muslim woman, a Jew or Christian. The 
    issue with regard to marrying them is that it is simply permitted in Islam. 
    Undoubtedly marriage to a righteous Muslim woman will help the Muslim to 
    obey his Lord and help in raising righteous children. 
Shaykh Muhammad ibn Saalih 
    al-‘Uthaymeen (may Allaah have mercy on him) said: 
A religiously-committed 
    Muslim woman will help him to obey Allaah and will be good to raise his 
    children, and protect herself in his absence, and protect his wealth and his 
    house, unlike a woman who is not religiously-committed, who may harm him in 
    the future. Hence the Prophet  (peace and blessings of Allaah be upon 
    him) said: “Choose the one who is 
    religiously-committed.” If along with religious commitment there is 
    also beauty, wealth and good lineage, then that is light upon light. 
    Otherwise, what he should choose is the one who is religiously-committed. 
    End quote. 
Al-Sharh al-Mumti’ 
    (12/13). 
Secondly: 
Marriage to a kitaabi woman 
    – whether Jewish or Christian – is permissible in principle, and there 
    should be no difference of opinion concerning that, because it is stated in 
    the Book of Allaah. But the Muslim should pay attention to the conditions of 
    that permissibility, and think of the consequences of that marriage. One of 
    the conditions of marriage to kitaabi women is that the woman should be 
    truly Christian or Jewish, because in the kaafir lands there are many women 
    who have no religion at all, or women who are Buddhists or follow other 
    idolatrous religions. The second condition is that she should be chaste, and 
    not a zaaniyah (one who engages in fornication or adultery); she should not 
    make friends with non-mahram men. Another condition is that guardianship of 
    the children should belong to the Muslim, and they should be raised in 
    accordance with the Book of Allaah and the Sunnah of His Prophet  (peace 
    and blessings of Allaah be upon him). 
Allaah says (interpretation 
    of the meaning): 
“Made 
    lawful to you this day are At‑Tayyibaat [all kinds of Halaal (lawful) foods, 
    which Allaah has made lawful (meat of slaughtered eatable animals, milk 
    products, fats, vegetables and fruits)]. The food (slaughtered cattle, 
    eatable animals) of the people of the Scripture (Jews and Christians) is 
    lawful to you and yours is lawful to them. (Lawful to you in marriage) are 
    chaste women from the believers and chaste women from those who were given 
    the Scripture (Jews and Christians) before your time when you have given 
    their due Mahr (bridal-money given by the husband to his wife at the time of 
    marriage), desiring chastity (i.e. taking them in legal wedlock) not 
    committing illegal sexual intercourse, nor taking them as girlfriends”
[al-Maa’idah 5:5]. 
Even though it is 
    permissible if these conditions are met, it is better not to marry her. 
The scholars of the 
    Standing Committee for Issuing Fatwas said: 
It is permissible for the 
    Muslim to marry a kitaabi woman – Jewish or Christian –if she is chaste and 
    free (i.e., not a slave), because Allaah says (interpretation of the 
    meaning): “Made lawful to you this 
    day are At‑Tayyibaat…”  
But not marrying her is 
    better and safer for the believer, lest she attract him and his children to 
    her religion. End quote. 
Shaykh ‘Abd al-‘Azeez ibn 
    Baaz, Shaykh ‘Abd al-Razzaaq ‘Afeefi, Shaykh ‘Abd-Allaah ibn Ghadyaan, 
    Shaykh ‘Abd Allaah ibn Qa’ood  
Fataawa al-Lajnah 
    al-Daa’imah (18/314, 315). 
See also the answer to 
    question no. 2527. 
With regard to the 
    consequences of marrying kitaabi women, there are many. We have mentioned 
    some of them in the answers to questions no. 
    12283,
    20227 and
    44695. 
Thirdly: 
With regard to keeping dogs 
    in the house and the prohibition on that, it is narrated in clear saheeh 
    texts. It is not permissible to keep dogs except for hunting, guarding 
    livestock, or farming. The Muslim knows that sharee’ah only forbids a thing 
    because of its evil and harm. 
All the Muslim has to do is 
    to respond to what his Lord has enjoined him to do or forbidden him. He 
    should know for certain that He does not prescribe anything unless it is for 
    a reason. Some shar’i rulings are purely the matter of worship, and we do 
    not know the wisdom behind them and the reason of some is rationally 
    understood. There is nothing to prevent us seeking the wisdom behind 
    rulings, but the Muslim should still heed the commands of Allaah.  
See also the answer to 
    question no. 9603 for important information. 
We are certain that the 
    prohibition on keeping dogs is in the Muslim’s interests, and that keeping 
    them is harmful and bad. Among the negative consequences of keeping them are 
    the diseases that they cause and that are transmitted via them, whether it 
    is from a vessel from which the dog has drunk, or in its faeces with which 
    the house in which it is kept is contaminated. This is what you can tell 
    this Christian woman if you marry her. 
It was narrated that Abu 
    Hurayrah (may Allaah be pleased with him) said: The Messenger of Allaah  
    (peace and blessings of Allaah be upon him) said: “The purification of the 
    vessel of one of you, if a dog licks it, is to wash it seven times, the 
    first time with dust.” Narrated by Muslim (279). 
The word “purification” 
    indicates that the vessel becomes impure (najis) if a dog licks it. This is 
    what has been proven by modern science, but we never doubted that it is 
    true. The vessel can only be purified by water and dust, and not any other 
    cleaning substance. This has also been proven by modern science, which 
    confirms that this religion is nothing other than revelation that was sent 
    to the Prophet  (peace and blessings of Allaah be upon him). 
See also the answer to 
    question no. 20939. 
It has been proven that 
    there are many diseases that may be caught by the person who keeps a dog, or 
    touches it, or kisses it. We have mentioned some of them in the answer to 
    question no. 69840. 
And Allaah knows best.