As Fr Stephen says, the key to understanding this film is not large amounts of bloodshed (though it has to be said Mel Gibson does seem to glory in gore), nor the violence, nor the wild animals jumping out of the screen at you. Nor is the key the evident anachronisms, including the fact that the "great" Mayan civilisation had "mysteriously" become all but extinct by the time the Spaniards arrived. The key is the quotation from the historian-philosopher Will Durant, given at the beginning of the film, "A great civilization is not conquered from without until it has destroyed itself from within".
What we see clearly in the film is the way in which the city culture, caught up in the hideous decadent pagan excesses of human sacrifice, but also the prevailing culture of death which devalues the weak and the old, has already within itself the seeds of its own destruction. There is no great city here, but a wearying environment where life does not flourish. A friend of mine, just last week, who has moved from Birmingham to York, said how difficult it is for so many people to live in our cities because they are so culturally oppressive.
We are living in an oppressive culture of death. Our society may not be so crass as pagan Mayan culture, but we see around us the evidence of the culture of death, most especially in the ever increasing bloodshed of the unborn who never see the light of day, because they are inconvenient, unwanted, or have simply been created in order to have cells harvested. Equally in the way in which we treat the world around us such that it may not be fruitful for our future generations (alluded to in the film).
We need to learn the lesson of Apoclaypto. Young people are the generation that needs to stand up against the culture of death and fight and witness for life. The problem always - just like in the Mayan civilisation - a civilisation which has been great but has fallen into decadence believes that it is still great. It does not realise how close it is to self-destruction. Do not be lulled into a false sense of security. Our culture of death cannot last, for it is against nature.
"I call heaven and earth to witness against you today that I have set before you life and death, blessings and curses. Choose life so that you and your descendants may live, loving the LORD your God, obeying him, and holding fast to him; for that means life to you and length of days, so that you may live in the land that the LORD swore to give to your ancestors, to Abraham, to Isaac, and to Jacob." (Deut 30.19-20).