This incident occurs in Chapter 18 of this text and is a wonderful example of the satire employed by Twain. In particular, Twain levels the guns of his satire against the hypocrisy that exists in so many Christian circles, when ostensibly churchgoers are meant to practice love and grace, but in reality their lives are very different. This is clear from the following description of the church service that Huck attends with his new hosts:
The men took their guns along, so did Buck, and kept them between their knees or stood them handy against the wall. The Shepherdsons done the same. It was pretty ornery preaching--all about brotherly love, and such-like-tiredsomeness; but everybody said it was a good sermon, and they all talked it over going home, and had such a powerful lot to say about faith and good works and free grace...
The bitter irony is clear. What strikes Huck as just being "ornery preaching" is actually incredibly pertinent. The two feuding families both come together to church and hear a sermon about love, which they say was a good sermon, and yet it has no impact whatsoever on the hatred that exists between the Grangerfords and the Shepherdsons. This is highlighted by the way that all the men bring their guns and either keep them with them throughout the service or lean them against the wall. Even though the sermon is all about "brotherly love," there is no evidence of any such thing in the actions of the two families.