An allusion is when an author refers to another work of literature or art.
In the beginning of Dr. Jekyll and Mr. Hyde, Robert Louis Stevenson describes one of the main characters in the story, Mr. Utterson. When one of his friends got himself into trouble, Mr. Utterson "was inclined to help rather than to reprove."
"I incline to Cain's heresy," he used to say quaintly: "I let my brother go to the devil in his own way."
This is an allusion to the Biblical story of Cain and Abel, found in the book of Genesis, Chapter 4. Cain and Abel are two sons of the first humans, Adam and Eve. Like many brothers who came after them, Cain and Abel quarrel, and Cain kills his brother, becoming the world's first murderer. When God asks Cain, "Where is Abel your brother," Cain replies, "I do not know; am I my brother's keeper?"
This answer is, understandably, usually interpreted as a cynical, cruel remark; after all, Cain himself murdered Abel, so it is no excuse to claim that he is not responsible for his brother's safety.
Mr. Utterson refers to this cynical, cruel answer as "Cain's heresy." What he means by this is rather an interesting twist. Whereas Cain truly did not care about his brother, Mr. Utterson truly does care about his friends. It's just that he does not rebuke them for their misbehavior which creates problems for them. Rather, he helps them.
Mr. Utterson displays this trait later in the story when he remains faithful to his friend Dr. Jekyll even after his behavior has become strange and other friends have abandoned him.