This movie has a lot of theological meanings. It challenges the popular notion to "see is to believe." It gives a strong case of how to describe faith in a very simplistic way. Simply because one cannot see a thing does not mean something does not exist. Horton strongly believes there are living beings in the speck although he cannot see them.
It also gives a strong illustration of faith. Horton holds on to his belief despite costing him to be banished from his society. He was persecuted. It will even cost him his life as the society clamored for him to give up his belief.
It also challenges the human arrogance that we are alon in this universe. In a sense, it adheres both to the idea that there could be other living beings other than us and there could be a greater being than us. This is what makes the movie interseting, Horton plays the powerful and bigger being that "holds the world in His hand."
Another interesting metaphor is the "tiny little voice" that most people refer to. For most, it could be our conscience. For Christians, it could be God. Well, God is not a tiny voice, so one can also see the metaphor of a Supreme Being talking from the heavens with a great powerful voice.
And then there is the ultimate sacrifice that Horton did to save the whole world - of the Whos. Sounds familiar? Christ came to save the world from the power of sin and death. Horton saves the world of the Who from instability and sure death.
This Christmas, this kind of movies is nice and a good starting point for children and youth to have a theological discussion.