The soul has followed Moses and the cloud, both of these serving as guides for those who would advance in virtue; Moses here represents the commandments of the Law; and the cloud that leads the way, its spiritual meaning. The soul has been purified by crossing the Sea; it has removed from itself and destroyed the enemy army. It has tasted of the waters of Marah, that is, of life deprived of all sinful pleasure; and this at first had seemed bitter and unpleasant to the taste but offered a sensation of sweetness to those who accepted the wood. Next it enjoyed the beauty of the palm trees of the gospel and the springs; it filled itself with the living water, that is, the rock. It took within itself the bread of heaven. It overwhelmed the foreign host - a victory due to the extended arms of the Lawgiver, which thus foreshadowed the mystery of the Cross. Only then can the soul go on to the contemplation of transcendent Being.
St. Gregory of Nyssa