What is the ruling on a Muslim woman who says there is no difference between Islam and Christianity, and is she thereby considered an apostate disbeliever from the religion of Islam?

A ruling of apostasy against a person can only be made after verifying that its conditions are met. The statement "there is no difference between Christianity and Islam" is false. Christianity was originally a valid religion, but it was abrogated by Islam, and nothing but Islam is now accepted from people, as Allah Almighty says: "And whoever desires other than Islam as a religion—never will it be accepted from him, and he, in the Hereafter, will be among the losers." (Quran 3:85) One of the fundamental differences is that one who turns away from Islam is considered to be denying some of the messengers, whereas the people of Islam believe in all prophets and messengers and do not differentiate between them, and they disbelieve in anyone who denies the prophethood of any prophet. As for the Christians, they deny the prophethood of Muhammad, peace and blessings be upon him, and they have polytheism, trinity, and the claim of a son for Allah.