Is it permissible for a woman who has recently embraced Islam to remain married to her Christian husband, while concealing her conversion from him and intending to separate from him later, and would she be sinful for doing so?

It is not permissible for a Muslim woman to marry a disbeliever or to remain married to one, due to the Almighty's saying: "And do not marry polytheists until they believe" and His saying: "If you ascertain that they are believers, then do not return them to the disbelievers; they are not lawful [wives] for them, nor are they lawful [husbands] for them." Therefore, she must separate from him and refrain from him, unless he declares his Islam during her waiting period (iddah). She is no longer his wife and must seek to free herself from him. She may conceal her Islam if she fears for herself, or she may claim to despise him and be unable to live with him in order to legally separate from him.