Good question, Jack. It is a question I have personally explored for myself, having arrived at the conclusion I am atheistic in my perspective only a few years ago.

I found myself cruising definitions of "atheist." Wikipedia seems to sum up the various definitions well enough.

Atheism, in a broad sense, is the rejection of belief in the existence of deities.[1] In a narrower sense, atheism is specifically the position that there are no deities.[2] Most inclusively, atheism is simply the absence of belief that any deities exist.[3] Atheism is contrasted with theism,[4] which in its most general form is the belief that at least one deity exists.[5][6]    full article here

I have, over the past several years, arrived at the position that I have an absence of belief that deities exist.

I also like Wikepedia's concise definition of belief, i.e. "the psychological state in which an individual holds a proposition or premise to be true.

I don't hold my premise to be true. I consider it a rational and logical conclusion based on the information available to me to the extent I have the tools and capacity to comprehend the information available to me.

I think concepts of good and evil are just that - concepts. From an evolutionary perspective they are essential constructs, especially for creatures who have evolved to be social creatures.   I don't think existence has any moral basis. I think it just is. I try to operate in the world from the notion that all existence has intrinsic value. I choose that only partially, because I am willing to eat, which means I am willing to kill other life-forms, be they animal or vegetable.

I think any moral imperatives are evolutionary in their origin and their function, and have to do with adaptation and survival. I think concepts of right and wrong/good and evil are relative because what it takes for a species to survive depends on an infinite number of variables. Beyond that, what it takes for an individual within a species to survive and perpetuate it's own particular genes adds another layer of complexity.

I have a very clear sense of values that govern the way I operate in the world. Most of these are inherited cultural and familial values. Many of them continue to make very good sense after examination, and I tend to operate from them pretty automatically, treating them as truth. I also have an innate drive to survive individually.

What I often experience as moral dilemmas arise out of the complex matrix of adaptation on a species-wide basis vs. adaptation to foster the continuation of my own particular gene pool. Most of my values are socially inherited and programmed. The vast majority of those values are ones I think most individuals and cultures in the world hold in common - at least within our own families, tribes, cultures.

All a very long-winded way of saying "What Ed T said."