Peace be upon you, sister!
It sounds like you are in a real tough spot. One’s faith can be a very significant variable in a marriage decision. It seems like each of you love and honor your relationship to the Divine which means you’re both devoted to your path. This is a great thing to have in common. However, in this case, while the paths have many similarities, they are not quite the same.
His options: (a) To marry you as a Christian or (b) not marry you because you are a Christian (and not a Muslim).
Your options: (a) Convince him that he can (according to Islam) marry a Christian woman even though he may prefer that you become Muslim. (b) Not marry him because you are unwilling to leave your church (which is your right to choose, of course).
Questions that, I believe, you should explore:
Why did he engage in a relationship with you in the first place knowing you are a devote Christian? Obviously, he knew there is a chance you would not become Muslim, yet he courted you? Did he ever voice at the beginning of the relationship that he would “only marry a Muslim woman” or did he spring this on you later? If you knew since the beginning he wanted to marry a Muslim woman, then you also took the same “risk” as he did and probably hoped for the best (that you would remain Christian and he hoped you would become Muslim).
Besides theological and ritual differences, both faiths have many values and beliefs in common. In interfaith marriages, the issues tend to stem from these theological and religious practical differences like holidays, cultural and social expectations, but primarily fear about the future family and upbringing of the children.
I would suggest you explore more of the reasons why he wants you to become Muslim and why he is not willing to marry you as a Christian (which he could). If it is a matter of theology or children? Try to unpack the fears and talk out potential solutions. These are the hard the discussions that must be explored in order to diminish any anxiety or fear about the unknown or possible future “headaches” that may occur.
Lastly, if these items are explored more thoroughly with no resolve, and both of you are still unwilling to compromise, then it may be an indicator that it is not going to work out. Sometimes, sister, “love is not enough.” Many people have been in love with the wrong people or people that will not be good for them in the long term.
A good criterion for a future partner is not just how well the relationship is now or today, but also how well a family with this person can be. Hence, people devoted to different faiths can hit a wall once marriage and future family are now variables to weigh on. Most of the time people like to go with what is familiar, comfortable, and predictable because it seems easier and less stressful.
I pray you find clarity and wisdom in your future choices. May the Lord give both of you what is best for you whether with each other or another, amin.
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