Saint Augustine of Hippo provides a classic interpretation of Psalm 91, which he sees as being an encouragement to courage and trust in God amid suffering and the snares of evil. He notes that the devil used this psalm to tempt Jesus, and so he gives it a Christological significance.
According to St. Augustine, this psalm not only gives encouragement to those facing evil or suffering, but also implicitly shows that those who fail to rely on God fall to these evil powers. In addition, he notes how God's protection extends not only to those who are going against visible and open means of resistance, but also against the quite, solitary suggestions of the devil.
Much of Augustine's exposition consists of interpreting the individual lines of the psalm. For example, he interprets the verses about the darkness of night as referring to sins in ignorance, and the daytime arrow as referring to sins committed with full knowledge.
St. Augustine draws further conclusions about the meaning and extent of God's protection by a comparison with the psalm's use by the devil against Jesus in Matthew 4. Here, Jesus rebukes the devil's use of the passage to put God to the test, and from this narrative St. Augustine further concludes that the protection guaranteed to those who look to God includes the humble, not the proud or those who trust in their own power.