References to God are never used by Macbeth, strengthening the concept of the fight between good and evil with Macbeth on the evil side.
Biblical references are known as allusions. There are actually a surprising number of direct references to God in the play, but Macbeth never makes any sincere references to God. One interesting one is in Act 5, Scene 8.
Why then, God's soldier be he!
Had I as many sons as I have hairs,(55)
I would not wish them to a fairer death.
And so his knell is knoll'd. (p. 89)
This line is spoken by one of Malcolm’s soldiers, Siward. He has just found out that his son was killed. His reaction is grief, of course. However, he also recognizes that Siward died defending God, because Macbeth represents evil.
The juxtaposition between references to God in the anti-Macbeth camp and people who are affected by Macbeth (such as Lady Macbeth’s doctor), and the lack of calling on God in Macbeth is interesting. Shakespeare is paralleling the fight between good and evil, with Macbeth and the witches on the evil side.
In fact, Macbeth seems frightened of the word “God” when he hears it. He assumes that God is, in fact, the other side. Look at his reaction right after he hears Duncan, when he thinks he hears someone use the word.
One cried, “God bless us!” and “Amen” the other,
As they had seen me with these hangman's hands.
Listening their fear, I could not say “Amen,”
When they did say “God bless us!” (Act 2, Scene II, p. 29)
Macbeth is worried because he cannot say “amen” when he thinks he hears them. This is an early indication that he has turned from the good side to the evil side, and even he is frightened by it at first.
Macbeth does not use God’s name, except as a social formality with little religious significance. He says “While then, God be with you!” (p. 41) to his guest in Act 3, Scene 1, because they expect him to. This one reference further underscores the point that Macbeth is Godless. He is uncomfortable referring to God, and he begins to realize that he is descending into devil territory.
Shakespeare’s audiences would have been aware of the conflict within good and evil present in the play, and none of the Biblical allusions would have been lost on them. Macbeth is not on God’s side.