It is clear that this episode from Chapter 18 of this novel is one of the more bizarre ones, as Twain uses this incident to highlight the massive hypocrisy of those who ostensibly are Christian and go to church but act in ways that are completely opposed to that belief. This impression is made even more apparent through the way in which both families take their guns with them and have them between their legs whilst they listen to the sermon. Note what Huck says the sermon is based upon:
It was pretty ornery preaching--all about brotherly love, and such-like tiresomeness; 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 and preforeordestination, and I don't know what all, that it did seem to me to be one of the roughest Sundays I had run across yet.
The irony is clear: not only is the sermon on the need for brotherly love, but the families find it a "good sermon" and discuss it throughout the rest of the day, being clearly blind to their own lack of "brotherly love" towards their enemies in the other family. The mood that is created, especially through Huck's description of this episode as being "one of the roughest Sundays," is one of disbelief and shock that people who are ostensibly so Christian in their beliefs fail to see in any way that they are not putting those beliefs into practice in their lives.