Hint 1: textuality of text: this is representation of ideals, not history of battlefield practice
God is all-powerful. God is all-good. These two statements are assumptions for the medieval mind. Characters in The Song of Roland assume that God will intervene in events; it seems perfectly reasonable to believe, for example, that deciding the verdict at Ganelon's trial should be done by combat, because God will supposedly aid the man in the right.
And yet, paradoxically, evil things happen. The poem manages to turn these events into part of God's plan. See the analysis of the seventh section of the poem for further discussion of this topic. There are other themes closely related to this, like duty, that you can check out in the link below.
http://www.gradesaver.com/song-of-roland/study-guide/major-themes/
