The main message of the epic poem Song of Roland is fairly simple: good will always triumph over evil. In this poem, "good" is represented through both honor and justice and always prevails.
In the main message, honor as a quality of goodness is quite important. The honor of simply being a "Christian" is stressed. Where Christians are seen to have "faults" (even major ones), they still have heaven ahead of them. There is no honor in being a pagan. Why? Pagans are destined for damnation. Roland specifically struggles with honor, especially in the horn-blowing incident. Roland finds dishonor in blowing the horn to call the entirety of French forces back to help him; however, Roland changes his mind after the guard is killed because honor requires the avenging of that death.
Again, goodness is always represented as justice. Because the Christians are on the side of Christ, they are always assumed to be right and on the side of justice. On the other hand, the pagans are not on the side of Christ and are therefore unjust and "wrong." Again, pagans (such as the character of The Babylonian Baligant) can be seen as having good qualities, such as being a valiant warrior, but pagans will always generally be considered unjust and on the side of evil. Further, Christians can be seen as having faults, such as too much pride, but they will still be recognized as on the side of justice and goodness.
In conclusion, it's important to note that this epic poem is about the crusaders for Christ and therefore has the same basic message: good triumphs over evil.