How did the righteous predecessors (Salaf Saliḥ) reach such a high degree of closeness to Allah, and how did they remove the worldly life from their hearts?

Two fundamental matters encapsulate the states of the Salaf that elevated them to high levels of faith: The first is their diligence and perseverance in following what the Prophet ﷺ brought. This is only achieved by learning and understanding what he brought from its authentic sources, then acting upon it. The second is the sincerity of their adherence, without any corrupt intention, and a certainty unmixed with doubt. Thus, they learned with a sincere intention and acted with unwavering conviction, striving against their selves to repel ostentation and worldly aims, committing to truthfulness, sincerity, asceticism in the world, and yearning for the hereafter. All of this depends on supplicating to Allah and seeking guidance and steadfastness from Him during times of response, such as the last third of the night.