We know the same thing

Christians know certain things. But atheists (espoused and practical) also know these things.

These are things about people that only make sense in the Christian understanding of the world and our experience in it.

Greg Koukl covers them in a blog and a podcast.

They are:

Humanity - 'man is different from non-man' (Schaeffer)

In a world without God. Humans are nothing but cogs in the celestial machine, cosmic junk, the ultimate unplanned pregnancy, left to build our lonely lives on the “firm foundation of unyielding despair,” as atheist philosopher Bertrand Russell put it. Nihilism—bleak “nothing-ism.”
Yet no one really believes this, not deep inside. Solomon said God has set eternity in our hearts.[6] There is a better answer—a more accurate answer—to the question “What does it mean to be human?” And we all know it.

 Two deaths: comparison of Mth Theresa and Diana, princess of Wales deaths

In a God-less universe where all meaning is of our own making, what could it possibly mean to say someone died an “untimely” death? It means that people know better. It means they know life has an ultimate purpose and deep significance that transcends private projects. In spite of their pontifications to the contrary, their mannishness gives them away.

It's broke

Here is another example—kin to the one above—of the “inside” truth-finding its way to the “outside.” Everyone knows something has gone terribly wrong with the world. We call it “the problem of evil,” and it prompts us to ask, “Why is there so much badness in the world?”
There is a wrinkle to this concern, though, another detail each of us also knows. The world is broken, true enough. But we are broken, too. Though humans have inherent dignity, we are also cruel. The evil is “out there,” as it were, but it is also “in here”—in us.

We all know guilt

There is something else, though. We are not left in despair, abandoned under the weight of blame we all share. “The answer to guilt is not denial,” I told them. “That’s relativism. The answer to guilt,” I said, “is forgiveness. And this is where Jesus comes in.”

Our restless souls

Two facts of the human condition lie at the heart of our inescapable sense of longing. One is that we are broken. We’ve already spoken of that. The second is this: It hasn’t always been this way. There remains a remnant of former beauty the brokenness cannot efface, yet something has gone missing that must be replaced. We feel a “sweet pain…a primal memory deep in our souls reminding us of the way the world started—good, wonderful, whole, complete.”