Reciting mantras is a tool for spiritual practice.

Question: After one has developed the habit of silently reciting the Śūraṅgama Mantra with a subtle holding in the depths of the mind, if a questioning doubt toward the Dharma arises in the depths of the mind, can one then replace the mantra with the questioning doubt, so that one is engaged in investigation whether walking, standing, sitting, or lying down? 

Answer: When a questioning doubt arises, one must concentrate single-mindedly on the investigation, without even the slightest distraction. Only in this way can the questioning doubt be effectively resolved and wisdom be brought forth. Nothing is more important than bringing forth wisdom. Reciting the mantra is also for this ultimate goal—it is merely a method of entering the Path. One must not take reciting the mantra as the goal of practice; it should be used as a tool instead.