How can one experience the sweetness of faith and closeness to Allah most of the time and maintain this feeling, even with the fluctuation of hearts? And is the "Nafs al-Mutma'innah" (the tranquil soul) the one that finds rest from the oscillation and suffering of faith?

Reforming the heart is among the greatest factors influencing the reform of a servant's condition. This is achieved by performing the obligatory duties, abstaining from forbidden acts, and then by increasing in supererogatory acts, along with fulfilling the obligations Allah has enjoined regarding the actions of the heart. This includes purifying it from destructive diseases such as rancor, malice, envy, and arrogance, and filling it with the love, fear, and hope of Allah. Among the greatest aids in reforming the heart is contemplating and understanding the Holy Quran, for it is a healing for hearts. It cultivates within them love, fear, hope, and all other states essential for their life and perfection, and it deters from blameworthy attributes and actions. If the heart is reformed, the soul becomes upright and tranquil, attaining an indescribable pleasure. The heart then bestows upon it garments of tranquility, serenity, and contentment. Therefore, a servant must care for their heart, continuously hold themselves accountable, and remain vigilant against the machinations of Satan and the soul that incites evil, for the battle with them continues as long as the servant is alive.