Thomas Brooks emphasizes the profound impact of secret duties, particularly private prayer, on the spiritual richness of believers. He compares secret meals that nourish the body to secret prayers that enrich the soul, suggesting that true spiritual wealth comes from intimate communion with God in private. Brooks argues that the most significant graces and experiences are cultivated in the quietness of one's closet, away from the distractions of public life. He illustrates that just as gentle dew nurtures plants more effectively than heavy rain, secret prayer fosters deeper growth in grace and holiness than public displays of faith, which can often be tainted by pride and hypocrisy.

Secret duties are the most soul-enriching duties. Look! as secret meals make fat bodies—so secret duties make fat souls. And as secret trades brings in great earthly riches, so secret prayers makes many rich in spiritual blessings and in heavenly riches. Private prayer is that secret key of heaven which unlocks all the treasures of glory to the soul. The best riches and the sweetest mercies, God usually gives to His people—when they are in their closets upon their knees. All the graces of the saints are enlivened, and nourished, and strengthened by the sweet secret influences which their souls fall under, when they are in their closet-communion with God. Certainly there are none so rich in gracious experiences, as those who are most exercised in closet duties. As the tender dew which falls in the silent night makes the grass and herbs and flowers to flourish and grow more abundantly than great showers of rain which fall in the day; so secret prayer will more abundantly cause the sweet herbs of grace and holiness to grow and flourish in the soul, than all those more open, public, and visible duties of religion, which too, too often, are mingled and mixed with the sun and wind of pride and hypocrisy.