is no longer a Christian nation?"
In a sense the statement has always been true. Note that in our first international treaty we stated, "... the Government of the United States of America is not, in any sense, founded on the Christian religion.." America has always had a plurality of worldviews. More Americans are Christian than any other religion but we are a multicultural nation. Note that the majority of Americans are Caucasians but someone saying, "America is a white nation" would be seen by most people as a bigot for not acknowledging people of color. What president Obama was saying is that America is a multicultural nation. Christians, Muslims, Jews, Hindus, Buddhists, Sikhs, atheists etc. all live in peace.
Well not everyone in America is a Christian. A majority aren't.