From: Letters from the REBBE volume 2 letter 19
Erev Succos, 5727
Blessing and Greeting:
I am in receipt of your Express letter, and although it is Erev Yom Tov and not very convenient for correspondence, I will reply as briefly as possible in view of the importance of the matter and your understandable anxiety, I leave it to you're discretion whether to show this letter to your sister, or convey to her its contents.
For a Jewish person to marry a non_Jew is one of the greatest calamities, and not only from the religious viewpoint. Nor is it entirely a personal matter affecting the person involved, for it concerns the whole Jewish people, and there are few transgressions which affect the whole Jewish people as an intermarriage, G-d forbid. It is a transgression also against one's elementary honesty, for it is exceedingly unfair to the other party, from the viewpoint of each, and it is also unfair to their respective good friends, who wish to see their near and dear one lastingly happy, an not otherwise.
It has often been pointed out that marriage in general, even between two persons of similar background, entails a certain risk as to eventual adjustment and compatibility. Even if the two had been acquainted for some time, it is no sure criterion as to what the relationship will be when the acquaintance is turned into a marriage, where the two will be thrown together under one roof for 24 hours in the day, day after day, and week after week, etc.
But when the background is entirely different, and where this deference dated back for scores of generations and consequently of a deep and lasting quality - the chances of adjustment and compatibility are negligible as to be non-existent. Especially, where the difference is of a definitely antagonistic and hostile nature, as has been evidenced by the pogroms and persecutions of Jews in Every land where Jews sojourned in the past 2000 years. Moreover, modern science recognizes the hereditary nature of character traits, particularly deep[rooted ones over generations.
Thus, if one is honest - in the plain sense of the word - one would not wish to drag another party into an alliance which is doomed from the start. And if one truly loves the other, and not in a selfish way, one would certainly not wish to involve the other into such a misfortune,and would readily forgo the prospect of immediate and short-lived pleasure in order to spare the other the inevitable result. Otherwise, the professed love is nothing but selfish and egotistic.
Should there be children from such a union, there is the added consideration of the tragedy of the children having to witness constant friction, and worse, between their parents, which are bound to follow in the natural course.
There is no need to elaborate on this very painful subject.
Needless to say, I am aware of the "argument" that the percentage of intermarriage is a considerable one and many of them seem to last. But it is sure unnecessary to point out that married people try to put on the appearance of a "happy" marriage, being ashamed to confess failure and to reveal the frictions and indignities, etc., suffered at home. In an intermarriage the sense of shame is even greater, knowing that many friends had warned them against it, and they had maintained that their marriage would be different. But as a matter of fact and statistics, the percentage of separations and divorces are incomparably grater than in non-intermarriages.
And another point. In the vast majority of cases, those that enter into an intermarriage are very emotionally invalid. Were thy themselves to be asked about others contemplating such a step, they would counsel against taking a step which would commit each other to possible lifelong misery. Indeed, they would consider it irresponsible to take such a step in an emotional state of mind.
As a postscript I wish to add, that according to Jewish Law the child goes after the mother. Therefore where the mother is Jewish, even if the father is not, the child is Jewish and duty-bound to fulfill all the Mitzvos, etc. Further details may be obtained from any Rov.
Wishing you and yours a happy Yom Tov,