My colleague Gopīparāṇadhana Prabhu notes, "Maṇi, mantra, and auṣadha [jewels, mantras, and medicine] are often grouped together by Vedic philosophers as examples of things in this world that have acintya-śakti (inconceivable energy)." Since the Supreme Personality of Godhead and His energies are inconceivable, it is understandable why the poets and philosophers compare Him to jewels and medicine and praise the wonderful powers of mantras composed of His names.
Aside from jewels, mantras, and medicine, every living being in the creation possesses acintya-śakti to some degree. Although human beings often consider themselves the most powerful of all God's creatures, many lowly creatures possess abilities far beyond those of human beings. For example, growing grass endures trampling and stays out all night in freezing weather without a protest. A human being is not so tolerant. Frogs possess an inconceivable ability to maintain their lives even while buried under the earth. Hummingbirds and insects are flying machines so sophisticated that they can outmaneuver airplanes in many ways. Although scientists tend to think of their work as demystifying the secrets of the universe, they admit that nature at its most basic level remains inconceivable.
The presence of acintya-śakti in both the smallest and the greatest aspects of the universe should lead us to ask, Who is the source of this inconceivable energy? That source is described in the Brahma-saṁhitā as the acintya-rūpa, or inconceivable form, of Lord Govinda, the Supreme Personality of Godhead.
The process of bhakti-yoga is also inconceivable. Every person fortunate enough to take up the process of Kṛṣṇa consciousness can attest to the inconceivable potency of the medicine of devotional service. Although we may have tried to give up vices before encountering Kṛṣṇa consciousness, we could not do so for long. But as soon as we began serving Kṛṣṇa and the pure devotee and chanting the Lord's holy names, the "impossible" was easily accomplished. The inconceivable energy that brings about these changes is called kṛṣṇa-śakti.
By the grace of the Supreme Lord, a pure devotee possesses this kṛṣṇa-śakti, and when he chants the holy name or speaks about Kṛṣṇa, the potent sound enters the consciousness of the receptive hearer and purifies him. By contrast, when a nondevotee speaks of Kṛṣṇa or chants His name, the effect on the hearer is not purifying but poisonous. Only one who is directly empowered by the Supreme Lord can spread Kṛṣṇa consciousness through kṛṣṇa-śakti. Thus only from the pure devotees can we gain the jewel, mantra, or medicine of devotional service, from which we can derive the inconceivable benefits of love of God.