One literal example of how Atticus was a "light in the darkness" comes when he stands (or sits, actually) alone against the lynch mob at the jail. Atticus knew it would be a long, if not dangerous, night, and he had the foresight to take a light bulb and extension cord with him so he could read.
In the light from its bare bulb, Atticus was sitting propped against the front door...
In ones and twos, men got out of their cars. Shadows became substance as lights revealed solid shapes moving toward the jail door. Atticus remained where he was. The men hid him from view. (Chapter 15)
Here, Atticus symbolically represented a "light in the darkness" of reason, and when the children appeared to stand by his side, reason won out, and the men returned to their cars, and once again hidden by the night, soon "they were gone."
Atticus's liberal-minded, color-blind approach to life served as an example to both his children and to citizens who were not yet filled with the hate and mistrust of Negroes. He taught Scout that using the "N" word was "common," and he was not upset (as Alexandra was) when he found that Jem and Scout had accompanied Calpurnia to her church; nor was he bothered by them being the only white occupants of the Negroes-only seating in the balcony of the courtroom. He gave Cal much more responsibility than most white employers allowed their black employees, and he takes on the Tom Robinson case even though he "had hoped to get through life without a case of this kind." He refuses to allow his children to "torment" Boo Radley, and they eventually come to recognize that he was not the nocturnal "ghoul" most of the town believed him to be. Atticus is the man people come to for advice, and as the local representative to the Alabama state legislature, he runs unopposed each election. Atticus would not change the way of thinking of everyone in Maycomb, but he did manage to take the town forward, if only in the form of "just a baby-step, but it's a step."