Jesus explicitly attributes their inability to the smallness of their faith (compare 6:30; 8:26; 14:21; 16:8), pointing out that even a mustard seed's worth of faith would be sufficient to cast out not merely demons but mountains (17:20; 21:21; 1 Cor 13:2). The disciples already recognized how small a mustard seed was (Mt 13:32). Ancient peoples thought of mountains as rooted far beneath the earth (Gundry 1982:353), so "moving mountains" was a typical Jewish teacher's image for doing what was virtually impossible. With this illustration Jesus indicates that even were we casting out mountains rather than demons, we would only be scratching the surface of a life of faith. What could we do with faith greater than that of a tiny mustard seed! Like children who have only begun to walk, most of us have only begun our adventure of faith.