NOTES for Heb 12:12-13
A quotation without quotation marks from the book of the prophet Isaiah points to the Messianic time that has arrived, the time when Christ, who has come into the world, points out and opens all paths. His paths are not our paths; they are straight. And our paths are not straight not because the road is crooked, but because we "swerve off course," and weakened legs cannot hold us steady. So Scripture calls us to focus our attention on the straightness of the path, to walk and through constant training strengthen our flabby muscles so that the skill of walking straight may arise. Of course, all this is a metaphor, where the path means life, and legs, knees, and muscles mean, above all, our will.