Psalm 91 is about the promises God gives those who trust in him: those who are in communion with God can depend on him for deliverance from danger and thus have constant serenity of mind. The believer is given over to the protection of God's angels.
In verses 1 to 8, the psalmist describes what God does for those who are in union with him, who "dwell" in him; namely, God protects the believer from any outside dangers. The believer does not need to fear either clear and present dangers nor unknown terrors. Even if the unbelievers are falling into doubt and fear, the believer can stand firm and upright in his union with God.
In verses 9 to 13, the psalmist says that God provides his angels to protect the believer. The angels keep the believer on the path he is trying to follow in union with God, and gives him power over the terrors of life, represented in the psalm by the lion and the cobra.
In verses 14 to 16, the psalmist speaks as with the voice of God, promising the believer deliverance from danger, honor from God and eventually salvation because of the believer's love for God.