What is the first thing to do to strengthen one's faith upon feeling it weaken?

Weakness of faith is a serious spiritual illness that leads to falling into sins, neglecting obligations, and hardness of heart. It is treated by addressing its causes, which include: distancing oneself from environments conducive to faith, lacking good role models, associating with transgressors, excessive preoccupation with worldly matters and children, prolonged hopes, and overindulgence in permissible things. The treatment involves shifting one's heart towards the Hereafter, reflecting on the Quran, and supplicating for steadfastness and renewal of faith.