What is the ruling on marrying someone who supports an excommunicating group, and is persistent in his view, knowing that he is not fully aware of the details of the excommunicating methodology?

It is permissible to marry a Muslim woman to a Muslim man, as long as he does not do anything that takes him out of the fold of Islam. If he does, then it is not permissible, due to the Almighty's saying: "And do not marry [your daughters] to idolaters until they believe." It is also important to seek out a man whose religion and character are pleasing, based on the Prophet's (peace be upon him) saying: "If someone whose religion and character you approve of proposes to you, then marry him. If you do not do so, there will be tribulation on earth and widespread corruption." If he supports groups with deviant ideologies, it is preferable not to marry him and to seek someone else whose religion and character are acceptable, fearing that he might corrupt the thoughts of his wife and children.