The Lord employs his power on our behalf, and moreover, infuses strength into us in our weakness. The psalmist, by an act of appropriating faith, takes the omnipotence of Jehovah to be his own. Dependence upon the invisible God gives great independence of spirit, inspiring us with confidence more than human.
7 The Lord is my strength and my shield;
My heart trusts in Him, and I am helped;
Therefore my heart exults,
And with my song I shall thank Him.
8 The Lord is their strength,
And He is a saving defense to His anointed.
The Lord is their strength. The heavenly experience of one believer is a pattern of the life of all. Jehovah is the same as he was to his servant David, “the least of them shall be as David.” They need the same aid and they shall have it, for they are loved with the same love, written in the same book of life, and one with the same anointed Head. And he is the saving strength of his anointed. Here behold King David as the type of our Lord Jesus, through whom all blessings come to us.
Taken from the Treasury of David by C. H. Spurgeon