I think that the story is titled "Salvation" because at the story's core it is about a boy's search and desire to find salvation. The narrator of the story is retelling an event that happened when he was 12 years old. That's a young and impressionable age. It's also still a young enough age where a lot of words and sayings are taken literally.
The narrator's Aunt Reed had been attending a religious revival all week, and she wanted to take the narrator to the last day. Aunt Reed told him that he would know Jesus had come into his heart and saved him when he saw a bright light. The narrator was the only child left and felt pressured into acting like he saw the light of Jesus. To everyone that was attending the revival, he was saved. He got his salvation. In that regard the title makes sense.
Even had the narrator not faked his bright light vision, I think the title still makes sense. The narrator is not anti-God. As that 12 year old boy, he's not even a skeptic. He wants to see the light and feel Christ's salvation. He's searching. And it shows a lot of strength of character that he holds out for so long against faking it. Many Christians talk about their spiritual journey and how their faith has matured over many years. Simply because the narrator didn't "see the light" that day doesn't mean he won't ever. That day ended with him thinking more about God and God's plan. That's a step in a long spiritual journey. Whether it ends with salvation or not is not told in the story, but the title still fits well.