He will cover you with his pinions, and under his wings you will find refuge; his faithfulness is a shield and buckler (ESV)
When I read this, I was reminded of Ephesians 6 and the whole armor of God. This Psalm might have been part of Paul’s inspiration for his phrasing, and reminds us that we should think a bit harder about the following passage. Its important to remember that the armor is indeed God’s.
Therefore take up the whole armor of God, that you may be able to withstand in the evil day, and having done all, to stand firm. Stand therefore, having fastened on the belt of truth, and having put on the breastplate of righteousness, and, as shoes for your feet, having put on the readiness given by the gospel of peace. In all circumstances take up the shield of faith, with which you can extinguish all the flaming darts of the evil one; and take the helmet of salvation, and the sword of the Spirit, which is the word of God, praying at all times in the Spirit, with all prayer and supplication. (Eph 6:13-18)(ESV)
The belt of truth is His Truth.
The breastplate of righteousness is His righteousness through Christ.
The helmet of salvation is His salvation, turning away His wrath.
The sword of the Spirit is, of course, His Spirit making known His words.
The shield of faith is really His own faithfulness to His promise, as Psalm 91 hints. And lest any man should boast, our faith itself is a gift from God (Eph. 2:8,9).
And what are we really preparing for with this armor? For a battle, yes. But we fight this battle on our knees. We pray. We speak His words back to him, and for what we miss, the Spirit will add its own groanings too deep for words (Rom. 8:26). We have a sword, but in our clumsiness, we dare not wield it while standing.
Finishing out the Ephesians passage:
To that end keep alert with all perseverance, making supplication for all the saints…
We put on the armor of God not so we can be independent machines of war, marching to glory and spreading the kingdom in our wake. We put on the armor of God so we can be even more dependent on God. We gird ourselves for war so we can ask our God to go to war for us.