Is the Dharma confirmed in the minds of sentient beings necessarily correct?

No, that is not so. If it were like this, the Buddha would not have told sentient beings: "Your mind is not to be trusted; only upon attaining arhatship may you trust your own mind." Here, arhatship refers to the ultimate fruit of the Small Vehicle, the fourth fruit; the third fruit still falls a bit short. Confirmation means that the manas definitively affirms and endorses something, forming uninterrupted attention (adhimokṣa of uninterrupted attention), penetrating to the marrow, unshakable, and also unfathomably deep. For example, with the self-view (ātma-dṛṣṭi), sentient beings' manas confirms this, and it is deeply rooted and unshakable—this is called self-grasping (ātma-grāha).

Trusting one's own mind without doubt and being extremely certain of oneself is the confirmation of the manas. Yet the confirmations of ordinary sentient beings (pṛthagjana) all carry great faults and entail great perils of birth-and-death. For instance, ordinary beings all confirm that the five aggregates (pañcaskandha) are the self; such confirmation is a wrong view (mithyā-dṛṣṭi), not right view. Since beginningless kalpas, ordinary sentient beings have always confirmed that their own views are correct and without error, yet throughout beginningless kalpas they have transmigrated through birth and death because of erroneous and inverted views, receiving all manner of evil and bitter retributions.

"Your mind is not to be trusted"—the Buddha used this not only to admonish ordinary sentient beings, but also to admonish those of the first, second, and third fruits. Before the fourth fruit, one's cognition of the emptiness of the world of the five aggregates is still not adequate or ultimate; the self-view in one's thoughts and concepts has not been fully eliminated, and self-grasping remains. Only the arhat of the fourth fruit has fully eliminated the self-view, is thoroughly emptied of it, and has eradicated self-grasping and the conceit "I am" (ātma-māna); only then is the conceptual framework of emptiness pure and may one trust one's own mind. This conceptual framework of emptiness is thorough and ultimate only within the scope of the Small Vehicle Dharma; compared with the Great Vehicle Dharma, it is still very incomplete and not ultimate.

Therefore, whether an ordinary being has attained realization or not, generally speaking, their wisdom is insufficient for self-examination. First, their period of practice is short and their experience inadequate; second, they have seen and heard little and have nothing to compare against, so misjudgment and erroneous judgment come easily. Moreover, because there is a self in their mind and they cannot transcend attachment, they will show favoritism toward and overestimate themselves, easily giving rise to false enlightenment and false speech—and in the end, cause and effect never fail.