Love. A Christian loves their enemies. The atheist is unable, literally unable to do so. It proves the difference in spirit.
As a Christian, you would have a different conception of love than I do. The Bible refers too love, but it doesn't really define it. In Christianity, the concept of love is tied to blood sacrifice.
John 3:16 defines love thusly: "For God loved the world so much that he gave his one and only Son, so that everyone who believes in him will not perish but have eternal life."
That same god that loves us so much is willing to keep the dead conscious just to torture them for not submitting to a book and the will of a priesthood.
As for loving your enemies, most Christians come nowhere close. "I hate the sin but love the sinner" has played out. Those words are justification for bigotry, especially homosexuals and atheists. Your own post expresses a bit of the latter.
My concept of love is nothing like that. Psychologically speaking, love is expanding the terrain that the self-loving instinct provides us. We begin by loving ourselves, and ourselves alone.
Love expands when we bring others inside of this circle of self and consider them as one does himself. If you love a wife, "I" becomes the two of you, and you do for her what you would for yourself, often putting her ahead of the singular self.
If children come along and you love them, too, you would treat them as the selfish and unloving treat only themselves. The circle of "I" expands further.
As one grows morally and spiritually, the circle expands further, perhaps including neighbors, other species, and strangers.
It's not just a rosy feeling. It's an incorporation of the other into the circle of self, and the protective and sacrificial behavior that follows from that conceptualization.
There is no place for blood sacrifice or torture there.
Would you like me to assemble the scriptures that show the love of atheists in your Bible, or the quotes from prominent Christians that belie that claim? You might agree with those scriptures and quotes, but it will undermine your claim that Christians love their enemies.
You also mentioned differences in spirit. Would you like to compare Christian and secular spirituality? It will be the same outcome. Up for it, or did you just prefer to make outlandish, unkind, marginalizing and demonizing comments about atheists?
Then we can move on to mercy and justice Christian and secularist style.