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

Is the dharma confirmed in sentient beings' minds necessarily correct? No. If it were so, the Buddha would not have told sentient beings: "Beware of trusting your own mind, for your mind is not to be trusted; only upon attaining the fruit of an arhat may you trust your own mind." Here the fruit of arhat 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 affirms and approves, forming uninterrupted mental engagement (adhimātra-manaskāra), deep into the marrow, unshakable, and also unfathomably deep. For example, regarding self-view (ātma-dṛṣṭi), sentient beings' manas confirms this, deeply rooted and unshakable—this is called self-grasping (ātma-graha). 

To have utmost faith in one's own mind without doubt, to be extremely certain of oneself—this is the confirmation of the manas. Yet the confirmation of ordinary sentient beings carries great faults and immense 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 painful retribution. 

"Beware of trusting your own mind, for 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 in place, not ultimate; the self-view has not been fully eliminated from one's thoughts and concepts, and self-grasping remains. Only the arhat of the fourth fruit has fully eliminated self-view, is thoroughly empty, and has eradicated self-grasping; only then is the mental concept of emptiness pure, and only then may one trust one's own mind. This concept of emptiness is thorough and ultimate only within the scope of the Small Vehicle dharma; compared with the Mahāyāna dharma, it is still far from thorough and 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 insufficient; second, they have seen and heard little and have nothing to compare against, so misjudgment is very easy. Moreover, because there is a self in their mind and they cannot transcend attachment, they will show favoritism and overestimate themselves, easily giving rise to false enlightenment and false speech—and cause and effect never fails.