There are scattered biblical allusions throughout "The Selfish Giant." There is a lush garden that represents the Garden of Eden, there is a Christ figure, and there is a crucifixion (of sorts). These allusions contribute to the story's overarching allegorical emphasis on Christian love.
The first allusion is the Giant's garden, whose natural purity makes it reminiscent of the Garden of Eden. However, this allusion doesn't suggest that the Giant's garden is exactly like the Garden of Eden—there are some major differences. In the Book of Genesis, Adam and Eve are expelled from the garden because they break the one rule they have been given when the serpent tempts them to eat the forbidden fruit. In "The Selfish Giant," the characters who are expelled from the garden—the children—are depicted as victims rather than transgressors. Moreover, he who expels them, the Giant, is not God. Quite the opposite, in fact: in his imperfection, the Giant is more like Adam and Eve than the children are. Rather than representing God, the Giant seems to represent humanity.
The second allusion is the little boy, who is quite explicitly identified as the Christ Child. The Giant is redeemed because he learns to love the children and because he demonstrates a wholehearted devotion to Christ. The little boy is the only member of the group of children with an individual identity. When the reader is first introduced to him, he is physically separated from the group. This is the first hint that he's not just an average child. His willing embrace of the Giant, despite the fear that this villainous character has instilled in the others, serves as another hint:
[...] and the little boy stretched out his two arms and flung them round the Giant's neck, and kissed him. And the other children, when they saw that the Giant was not wicked any longer, came running back, and with them came the Spring.
The little boy acts as a sort of leader for the children, despite his separation from them. Later in the story, the reader learns that the children "did not know where he lived, and had never seen him before." Nonetheless, they reflexively follow his example and behave as his disciples; it is the little boy who indicates to the children that the Giant has been redeemed. It is not simply a matter of the Giant changing his ways, as the children have yet to trust him. Rather, the shift in the Giant's character development and the story's overall plot hinges on the forgiveness granted to the Giant by the little boy, which he performs through the embrace and kiss—an act that is quite Christ-like.
The third allusion revolves around the little boy's wounds, which refer back to the crucifixion:
For on the palms of the child's hands were the prints of two nails, and the prints of two nails were on the little feet.
Witnessing the stigmata on his friend greatly upsets the Giant, but the little boy reassures him that "these are the wounds of Love." The Giant, in his sinfulness, represents humanity, and the little boy is the Redeemer. Supposedly as a result of his redemption of the Giant, the little boy has been crucified. He returns in order to bring the Giant with him to his garden, "which is Paradise." Just as humanity’s expulsion from Eden leads to its subsequent redemption through Christ, then, the Giant's wicked ways ultimately lead him to his redemptive encounter with the little boy.
