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