How did we go from being a nation that was Christian to one that is full of Muslims, atheists, and Buddhists?
First of all, you are badly overstating the degree to which Christianity has lost popularity in the United States. Americans are much more devoutly religious than the people of any other Western country. More than 78% of Americans identify as Christian while 15% identify as no religion. With 5% refusing to answer, that leaves only about 5% of the population that belongs to any non-Christian religion, including Judaism. So the US is hardly "full of" the religions you mention.
To the extent that we are "full of" these other beliefs, it is because we are a nation of immigrants and because we are (thankfully) still a nation with freedom of religion. Immigration brings people of other religions to our country. Freedom of religion allows them to keep their religions and also protects people who do not have a faith.
America is in no danger of being anything but a majority Christian country.