Psalm 91 is about trust and confidence in God. It opens with an invitation addressed to one "who dwells in the shelter of the Most High" (i.e., the Temple) and describes the condition of him who habitually trusts in the Lord (v.9). The invitation is to invoke the Lord with the words: "My refuge, My fortress; God in whom I trust." (v.2) Following the invocation is a didactic part where the psalmist gives assurance of the protection of Yahweh (vv. 3-4) the security that this affords (vv. 5-8), the safety it provides even in the face of danger (vv. 10-13). At the end of the didactic section, Yahweh answers the one who has made the invocation (v. 2). The New Living Translation renders this well when it inserts "The Lord says" at the beginning of this section in v. 14. Yahweh's response is a first-person confirmation of what has been said in the didactic part of the psalm (vv. 3-13), and more. He will not only rescue His devotee, but will also set him on high, glorify him and give him long life. In other words, He will make him taste "the power of (His) salvation" (v. 16, NIV).
The central controlling idea of the psalm is found in the didactic section, i.e. in vv. 9-10.
It is not enough to know that God is a refuge and a fortress; one must make Him one's refuge and fortress by a conscious choice. This is expressed in the use of "ki" in verse 9. It is not only emphatic here ("Indeed, surely" as in verse 11); it expresses a reason that explains the devotee's security in the face of evil and affliction. Verses 3-8 identifies the evils and afflictions from which the devotee is rescued and protected: the fowler's snare, the devastating plague, the terrors of the night, arrows that fly through the air... These are images of war, disease and the threat of enemies that make life insecure. These also typify the punishment that befall the unfaithful and the wicked. Since it is God who shelters and gives refuge, the devotee will have no fear (v. 5). In fact, the psalmist even emphasizes the devotee's security by telling him/her that destruction will be but a spectacle as far as he/she is concerned: "Just look around and watch," he says, "and you will see how the wicked is punished." (v. 
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The psalmist uses the image of a large bird giving protection in order to illustrate how Yahweh protects His own:
He will cover you with His feathers
Spread His wings that you may be safe (v.4)
There are examples in the Scriptures where Yahweh is likened to a bird nurturing its young (cf. Psalm 17:8, Deut. 32:11). In the NT, Jesus applies to himself to image of a mother hen who wants to protect its chicks (cf. Mt. 23:37; Lk. 13:34).
Verses 11-13 is familiar to NT readers because it is quoted by the Devil as he tempts Jesus to worship him (see Luke 4:1-13). It is a Jewish belief that Go sens His angels to protect a devotee. This belief is expressed literarily in the book of Tobit where the angel Raphael is featured. Jesus too speaks of the "little ones" having angelic advocates (Matthew 18:10).
God's Providence will be such that even when the devotee finds him/herself in life-threatening situations, he/she will not be harmed. Daniel was thrown into the lion's pit but was unharmed. Paul was bitten by a venomous snak but was unaffected (Acts 28:3-5). In other words, the psalmist seems to say that he who trusts in God will have peace. He thus prepares for the words of Yahweh in verses 14-16.
Indeed, I will deliver
whoever clings to me
I will set on high
him who knows my name;
I will answer
him who call upon me
I will be with him in distress
Him, I will deliver and glorify.
With length of days I will satisfy him
and I will show him my salvation.
The devotee is one who clings to Yahweh, knows His name and calls upon Him. "To cling" characterizes the loving relationship between husband and wife (cf. Gen. 2:24) and so some translations render v. 14 as "whoever loves Me". The "Name" of Yahweh is not simply a label: it is He Himself. To know His Name is to be acquainted with Him and the way He deals with men and women. The act of calling upon Him is the expression of the love and knowledge that one has of Yahweh and flows from a heart-felt relationship with Him. Yahweh takes care of His own and stays with Him. The assurance that "I will be with him distress" is a declaration that "I AM" is totally dedicated to the devotee.
