"They may not marry....." [Emor 21:7]
Letter from the Lubavitcher Rebbe zt"l
By the Grace of G-d
19th of Sivan 5717
Greeting and Blessing
This is in reply to your letter in which you write about the case of the Kohen who is contemplating marrying a divorcee. I am surprised that there should be any doubt on the part of any Jew about the strict prohibition of such a marriage, inasmuch as it is emphatically prohibited, both in the Written Law as well as in the Oral Law. So strict is the prohibition, that a kohen who violates this law desecrates his sacred calling, which is his heritage of countless generations.
The point I do wish to emphasize here is that in all matters of matrimony, the happiness of two partners is involved, and if there is any issue, the happiness of children and future generations is at stake. Obviously a marriage which has been prohibited by the Creator and Master of the Universe is one that cannot possibly be a happy one, and is certain to be harmful to both parties concerned.
In other words, if the said kohen has any feelings for the divorcee in question, he should realise that his marrying her would expose her to untold harm, not only in the afterlife and in a spiritual sense, but also in this life, and even in a physical and material sense. The fact that this may be beyond one's comprehension is immaterial, for it is certain that the Creator of the world knows best what is good for His creatures, and since He has so strictly prohibited such a marriage, there can be no doubt that it is harmful. Therefore, even on humanitarian grounds, the said kohen, if he has any feeling for the said divorcee, should give up the idea and avoid causing himself and her irreparable damage, physically and spiritually.
I trust that you will find the suitable words to explain the seriousness of the matter to the person in question, which no words can really overemphasize.
Also see: Kohanim and Forbidden Marriages