Artur Weiser, in his seminal Old Testament Library Commentary on the Psalms (SCM Press, 1962), writes that spiritual assurance is "the indestructible energy of a life fed by the invisible resources of communion with God." In Psalm 73, for example, which begins with the psalmist's confession of weariness and frustration over the prosperity of the wicked, it is not until he enters the sanctuary of God that he truly perceives their sin and his own.
As long as he was enamored by all the things the wicked seem to have that he didn't have he was filled with nothing but cynicism and anger. He had not yet realized that the life of those people was as filled with as much sorrow as his own. Short of turning to God their way of living was "like a dream" that when one awakens from it "you despise their phantoms."
And not only so. On beholding God's glory he himself was transformed. As if for the first time, he sensed that God had been standing beside him in the company of the faithful, holding his right hand even while his eyes were dimmed and his heart was hardened. Though he himself had forsaken God--acting "like a brute beast" toward him, "stupid" and "ignorant"--God had been standing beside him the whole time, holding his right hand.
The vital energies now transforming him are different from what material pleasures he so long envied in others could ever have provided. His whole life now rests on a new foundation, and "its wealth consists in the inner possession of opportunities of life provided by God."
The life of faith, we learn, is not bound to circumstance. It is an inner thing, lit from within us by the Holy Spirit, sovereign over every darkness known to human beings, "the indestructible energy of a life fed by the invisible resources of communion with God."