what I believe in

I am sure I am a member of atheists,and often I do things which I believe can bring me more benefits.
However, an experience confused me, why I performed in that way.

One day, when I bought a gift for someone, I gave the cashier 50 yuan for the present. To my surprise, she who’s talking to another one, change me another 50 yuan unexpectedly. I was confused: she should not give me back, for the gift cost me exact 50 yuan!
To be honest, at that moment, I was hesitate about what to do. But I returned the money back and told her that she made a mistake in the end. So I gave up a chance of “earning” 50 yuan.
In fact, I didn’t feel happy, ashamed or regretful later. I was just thinking why I deal with the money in that way, not taking it away.

I told this to my friend. He interrupted me in a half, “Did you take the money away?” “No. Though I had thought of that.” “Haha. You performed well. The money did belong to you exactly, so you would lose something if you did take the money.”
“Er? You are an atheist, aren’t you?”
“…… Yes.”
“But what you said……”
“Well, we don’t believe in God, or someone or something blessing us. But it doesn’t mean that we believe in nothing, does it?”

I understand my reason and my friend’s words after I waked up on one morning.
Though the Bible doesn’t tell me my dos and don’ts, my value, including conscience, other than thinking about visible benefits, is guiding me.