An atheist is a Christian who goes one further.
I find that is one of the stupider "clever lines" for atheism. "Well, you don't believe in these gods, so why not go one further?"
Because there are two different questions. The first being "Is there a Divine reasoning behind existence?" which if the answer is yes then leads into the second question "How is the Divine governed?" Just proclaiming "I just believe in one less than you!" is not an argument, because of the differences of viewing the world just from those two different viewpoints. Polytheists can easily understand monotheists and the reverse, usually the difference is in how they view ins and out of spiritualism and how the Divine should be properly worshipped.
Atheists, or at least ones that don't follow something like Buddhism or something similar, outright deny the spiritual. So no, an atheist is not "a Christian who goes one further."
It is an inherently stupid thing to state, because it does not even address the issue of "Is there something Divine in existence or not?" Unless you are spiritualist atheist (and even then it could be argued), the "in believing one less god than you" line does not work, because believing in just one god fundamentally changes the way you view the entirety of existence.
If you are going to claim that all of western morality descended from Christianity (which is dubious), you need to also accept that so did Socialism, for all it tries to reject Christianity.
This is true, it basically tried promising Heaven on Earth, and was at least partially based on the Christian views of "equality" or "fairness" kind of values, except taken in a completely materialistic way, with spirituality being either inconsequential or outright in the way of the "Glorious Revolution!"
Thus is a way, Communism and Socialist thought could be considered a Christian Heresy. One that outright denies God rather than misconstrues him, at least when they are the type to go "Religion fundamentally gets in the way of the Revolution!"