The argument between Beneatha and her mother Lena concerning God shows their radically different ways of looking at the world. Lena holds fairly traditional views, which was common with most women of her time. A devout Christian, she is someone who attributes all the good things in life, including Benetha’s future career as a doctor, to divine intervention.

Beneatha’s pretty fed up with her mother constantly invoking God. With her rationalist mindset, which is undoubtedly what has led her to want to pursue a career in medicine, she has no time for what she perceives as a childish, superstitious outlook on the world. For her, God isn’t real; he’s just an idea, an idea that she simply cannot and will not accept. Beneatha’s rejection of the Almighty is truly shocking and disturbing to Lena, but as Benetha rightly points out, this doesn’t mean that she’d going to go out and be immoral or start committing crimes. Her humanist values lead her to want to care for other people, not hurt them in any way.

But Lena can neither understand nor accept any of this. She responds by slapping Beneatha right across the face, a clear indication that this is her house, and that in her house there will always be a place for God.

During a conversation concerning Beneatha's future, Beneatha comments that she will become a doctor before she gets married, and Lena responds by saying, "’Course you going to be a doctor, honey, God willing." Beneatha proceeds to tell her mother that God has nothing to do with it and says she is sick of hearing about God all the time. She then asks her mother if God pays tuition and says,

Mama, you don’t understand. It’s all a matter of ideas, and God is just one idea I don’t accept. It’s not important. I am not going out and be immoral or commit crimes because I don’t believe in God. I don’t even think about it. It’s just that I get tired of Him getting credit for all the things the human race achieves through its own stubborn effort. There simply is no blasted God—there is only man and it is he who makes miracles!

When Lena tells Beneatha that God willing she will become a doctor, Beneatha responds by saying "God hasn’t got a thing to do with it" (Hansberry, 53). Beneatha then proceeds to criticize the idea of God by saying that she is sick of hearing his name and even asks if he pays tuition, which upsets Lena, who threatens to slap her daughter. Beneatha then expresses her genuine feelings regarding spiritual matters by denying God's existence and telling her mother that she does not accept the idea of God. Beneatha illustrates that she is adamant about her beliefs by saying,

There simply is no blasted God—there is only man and it is he who makes miracles! (Hansberry, 53).

Beneatha is a realist who believes in tangible, scientifically provable concepts and completely denies the existence of God. Beneatha believes that men and women should be championed and given credit for accomplishments instead of always giving God credit. Beneatha's heretical beliefs dramatically contrast with Lena's religious beliefs, and she ends up getting slapped across the face for denouncing God.

Mama Younger, the strong matriarch of the extended Younger family all living in one tiny, slum-like apartment, is a devout, if traditional, Christian woman.  She rules the household, including prohibiting the Lord's name from being taken in vain, for instance.  Her progressive-minded daughter Beneatha, however, wants to train to be a medical professional, and rebels against her mother’s more traditional values. 

As the stress in the household grows, Beneatha has an argument with Mama Younger, crying out,

I'm just tired of hearing about God all the time. What has He got to do with anything?... I'm not going to be immoral or commit crimes because I don't believe. I don't even think about that. I just get so tired of Him getting the credit for things the human race achieves through its own effort. Now, there simply is no God. There's only man. And it's he who makes miracles. 

In other words, Beneatha puts her faith in a man (or woman!) to make decisions that determine his own fate.  This sets up the ultimate decision the family makes, to move into a house in the white neighborhood and become part of the tide of change that is coming along with the American Civil Rights Movement.