Atheism is a negative statement. It is lack of belief in a god, or, a belief that gods do not exist. Contrasted to a theist, a positive statement, a belief in God, a belief in something. If someone claims to be an atheist, the only thing one can determine about them from that statement is that they do not believe in a god. They may be moral or immoral or amoral, etc. etc. They may be anything, except a believer in gods.
As far as the case for morality goes, morality should not be a ball and chain around your ankles (hampering your progress in life), nor a threatening whip above your head (do these things or you will be punished). Morality should be used as a guide to improve and sustain our lives.
Consider a plant. A plant, like a human, needs certain things for its survival. Air, water, sunlight, soil. But a plant has no choice in whether or not it gets these things. It is rooted in place, without volition, without cognition. Plants act automatically to gain the things they need to survive; there is no such thing as a "moral" or "immoral" plant. Even lower animals act automatically to survive. Would you judge a lion as immoral for chasing down and killing a gazelle?
It is only with rational, volitional animals where morality can apply, when a choice between life and death can not only be made, but understood; and it is the needs of such animals (humans) that must be served by morality. In such a view, morality is not subjective, based off of a person's (or society's) whims, nor is it given by an omniscient, omnipotent being. It can and should be determined objectively, through reason, by looking at what an individual needs to further his or her own life, and acting accordingly.
Consider for a moment all of the achievements of men that have been made through faith, and all of the achievements of men that have been made through force. Then contrast these to all of the achievements of men that have been made through reason. Now tell me which of these three has had a greater hand in furthering human existence.