David's light, salvation, and stronghold is the Lord. During David's time of need, the Lord was his salvation from his enemies. In spite of his military prowess, David never forgot that his ultimate victory was due to God. He realized that he had nothing to fear when he considered the Lord filling these roles. Likewise, we do not need to fear because the Lord keeps us safe.
In this great hymn of triumph, we are declaring our trust in the Lord to save us. There is no doubt that David's proclamation should be recited by all the saints of God. This is because the Lord Jesus is the light of the world, and in Him is life eternal, as well as safety and strength, purity and peace. He provides security and salvation for all who believe. He has conquered sin and death through His sacrifice.
As we are guided and led by His eternal light, we are filled with the divine light of a heavenly birth. The light of His love dispels all darkness, soothes all sorrows, calms our fears, and directs us to the path of righteousness and truth. Not only is He our light and salvation, but He is also our strength, our peace, and our God.

Dear Heavenly Father, thank You for Your faithfulness throughout the ages. Thank You for Your steadfast love that has blessed each generation, giving hope in dark times. In You, I have strength and sufficiency, light and life, my all-encompassing existence. As dark clouds loom across the sky of my heart, may I immediately remember the wonderful truth of Who You really are. Let me rest in the wonderful security and eternal salvation that You have provided. 
In Jesus's name I pray. Amen.