All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger. We ask Allaah to exalt his mention as well as that of his family and all his companions.
If one of the marriage partners apostates from Islam, then there are two cases:
Firstly, if one of them apostates before consummating the marriage, then the marriage contract becomes void according to all the scholars. Allaah says (interpretation of meaning): {…hold not the disbelieving women as wives.}[60:10]. Allaah also says about the believing women who come as emigrants (interpretation of meaning): {O you who have believed, when the believing women come to you as emigrants, examine [i.e. test] them. Allah is most knowing as to their faith. And if you know them to be believers, then do not return them to the disbelievers; they are not lawful [wives] for them, nor are they lawful [husbands] for them.}[60:10]. So if an apostate re-embraces Islam, a new marriage has to be conducted.
Secondly, if one of the marriage partners apostates after the marriage is consummated, then they have to be separated according to all the scholars. However, they differed whether this separation is a divorce or a nullification of marriage, and whether it takes immediate effect or after the waiting period is finished. The Shaafi'ee School are of the view that the separation does not take place except after the end of the waiting period and provided that the apostate does not embrace Islam. They also said that when the waiting period ends, separation takes place as a nullification of marriage not as divorce. They do not marry again except with a new contract but the separation is not counted as divorce. Other opinions have been reported in this regard. Anyway all of them agreed that if the waiting period is finished and he did not become a Muslim, then the marriage is nullified and she does not return to him except after a new contract and after he becomes a Muslim.
Allaah knows best.