He is in love with a Christian woman at the expense of his wife
Question

                A young man is in love with a Christian woman for whom he cares, but he feels guilty. He wants to leave his first wife (the mother of his children) and call this Christian woman to Islam, then when she becomes Muslim he will marry her.

Leaving his first wife does not mean divorcing her, lest that affect his young children. His first wife is very angry and upset.
Praise be to Allah.
    Firstly: 

    The questioner has done very well by seeking to advise his 
    friend in the light of Islamic teachings. This is an example of perfect love 
    and Islamic brotherhood. The Prophet  (peace and blessings of Allaah be 
    upon him) said: “Religion is sincerity (or sincere advice).” (Narrated by 
    Imam Muslim). If such good care was widespread among the Muslims their 
    situation would be far better than what we see now, especially in countries 
    where Islam and its followers are strangers. The Muslim is in great need of 
    his Muslim brother to give him advice and to love for his brother what he 
    loves for himself. Perhaps a kind word or sincere advice that a man gives 
    and pays little attention to may earn him the pleasure of Allaah, which will 
    save him from eternal misery and doom. 

    If your friend really wants to know the truth, he has to be 
    honest with himself and free himself from whims and desires, especially in 
    such situations. 

    In the question itself there is the admission that his love 
    for that Christian girl is a sin. How can it be acceptable for a Muslim man 
    to love a woman in this way, when she is not his mahram and not his wife, 
    and it may be said to him, this woman represents the honour of another man. 
    Just as you would not accept for a non-mahram man to love your daughter or 
    your sister when he is not her husband, then others would not accept this 
    from you. 

    In such cases of love, the only thing that is permissible is 
    marriage based on the sound principles of sharee’ah, not just love which the 
    questioner himself accepts is a sin. When our pure monotheistic religion set 
    out regulations governing the relationships between men and women, it set 
    out a proper way for these relationships to be conducted. So marriage is the 
    beginning of these relationships not the result thereof. Marriage is not the 
    result of love which is a sin. Moreover, this system of relationships is in 
    the interests of the Muslim society, and its stability and soundness, 
    because going against the teachings of Islam leads to a great deal of 
    corruption and mischief, such as that mentioned in the question which causes 
    grief to the first wife. 

    One mistake leads to another, and another, and so on. This 
    young man – may Allaah bless him in this world and in the Hereafter – wants 
    to correct this sin by forsaking his first wife, but this is another sin 
    which is no less serious than the first one; a sin cannot be corrected by 
    means of another sin. 

    When Islam permits plural marriage, it stipulates the 
    condition of fair and just treatment for all the wives, so it obliges the 
    husband to treat all his wives fairly. Allaah says (interpretation of the 
    meaning): 

    “but if you fear that you shall not be able to deal justly 
    (with them), then only one or (slaves) that your right hands possess”

    [al-Nisa’ 4:3]

    Undoubtedly it is not just or fair to forsake the first wife 
    in order to marry the second, even if we assume that the intention in this 
    case is good, which is to make the Christian woman embrace Islam, for the 
    Islam of the second wife does not require that the first wife be mistreated. 
    The end does not justify the means. Moreover, there are many ways and means 
    of calling the Christian woman to Islam, all of which may be used. 

    How many people have been called to Islam without that 
    involving wrongdoing or committing of sin. 

    Based on the above, there can be no doubt that his first wife 
    will get angry; if he were in the same position he would get very angry 
    too. 

    Read with me the following hadeeth: “Whoever has two wives 
    and is more inclined towards one of them will come on the Day of 
    Resurrection with half of his body leaning.” (Narrated by al-Tirmidhi, 1141; 
    al-Nasaa’i, 3942; Abu Dawood, 2133; it is saheeh according to Irwa’ 
    al-Ghaleel, 7/80, no. 2017) 

    Would you like to come on the Day of Resurrection with half 
    of your body leaning? Undoubtedly you would not like that. 

    All of the above applies if he wants to forsake his first 
    wife for a limited period of time. However, if he wants to forsake her 
    permanently, then it is even worse. I do not know which is meant in the 
    question. 

    Let us assume that you marry this woman after she becomes 
    Muslim, then you come to know of another woman who, if you marry her as a 
    third wife, that may be the reason for her becoming Muslim, but you do not 
    know her or love her. Will you forsake the second wife so that this one will 
    embrace Islam even though you do not love her? Answer yourself honestly. 

    Finally, I do not advise you to marry this Christian woman in 
    such circumstances where the matter is based on a previous sin, which is 
    loving her unlawfully. Yes, you are to be commended for your eagerness to 
    guide someone else to Islam. You could give her some Islamic books, 
    publications and tapes, and tell her about Islamic centers and female Muslim 
    daa’iyahs, or try to get her married to someone who is committed to Islam 
    and who could help her to remain steadfast, but without wronging a previous 
    wife, etc. 

    Secondly, it is obligatory to treat wives fairly with regard 
    to spending nights with them and spending on them. It is not permissible to 
    forsake the first wife unlawfully, even if that is with a noble intention. 

    Thirdly: The Muslim who is keen to keep his religious 
    commitment sound will keep away from love that is not based on a sound 
    foundation, even if that has a strong effect on the heart at first. Allaah 
    says (interpretation of the meaning): 

    “As for those who strive hard in Us (Our Cause), We will 
    surely guide them to Our paths (i.e. Allaah’s religion — Islamic 
    Monotheism).”

    [al-‘Ankaboot 29:69]

    And the Prophet  (peace and blessings of Allaah be upon 
    him) said: “There is nothing that you give up for the sake of Allaah, but 
    Allaah will compensate you with something that is better for you than it.” 
    (Narrated by Imam Ahmad, 20222; classed as saheeh by al-Albaani, 
    al-Silsilat al-Da’eefah, 5)

     You should strive to attain the mercy of Allaah and Paradise 
    by being kind towards the mother of your children and pleasing her as you 
    have angered her, and trying to make her happy after you have made her sad, 
    after she has lived with you for more than seven years.

     May Allaah help you to do all that is good.