In Chapter One, we are presented with Huck's views on what happens to you after death and his own personal preference, which, with his child-like logic, is actually incredibly amusing. Having been biblebashed by the Widow Douglas, Huck decides that it is more important for him to be where his best friend is. As Widow Douglas tells him that Tom Sawyer is not going to get to heaven, he personally opts to go and be with Tom Sawyer. Note how he reaches this conclusion:
Now she had got a start, and she went on and told me all about the good place. She said all a body would have to do there was to go around all day long with a harp and sing, forever and ever. So I didn't think much of it. But I never said so. I asked her if she reckoned Tom Sawyer would go there, and she said not by a considerable sight. I was glad about that, because I wanted him and me to be together.
Notice how Huck isn't actually rejecting Christianity. His understanding of religion clearly isn't very sophisticated. Instead, Huck is just ambivalent towards religion, which represents to him another aspect of the civilising force of society that he is so desperate to escape.