Srila Jiva Gosvami explains diksa in his Bhakti-sandarbha (283):

"Even though born in a brahmana family, one cannot engage in Vedic rituals without being initiated and having a sacred thread. Although born in a brahmana family, one becomes a brahmana after initiation and the sacred thread ceremony. Unless one is initiated as a brahmana, he cannot worship the holy name properly."

"Unless one is initiated by a bona fide spiritual master, all his devotional activities are useless. A person who is not properly initiated can descend again into the animal species."

"It is the duty of every human being to surrender to a bona fide spiritual master. Giving him everything-body, mind and intelligence-one must take a Vaisnava initiation from him."

"By chemical manipulation, bell metal is turned into gold when touched by mercury; similarly, when a person is properly initiated, he can acquire the qualities of a brahmana."

"In the morning, afternoon and evening, one should worship the Deity, chant the Hare Krsna mantra, offer oblations, perform a fire sacrifice, and feed the brahmanas. These five activities constitute purascarya. To attain full success when taking initiation from the spiritual master, one should first perform these purascarya processes."

"Without performing the purascarya activities, one cannot become perfect even by chanting this mantra for hundreds of years. However, one who has undergone the purascarya-vidhi process can attain success very easily. If one wishes to perfect his initiation, he must first undergo the purascarya activities. The purascarya process is the life-force by which one is successful in chanting the mantra. Without the life-force, one cannot do anything; similarly, without the life force of purascarya-vidhi, no mantra can be perfected."

Of similar importance is diksa, which is explained as follows in Bhakti-sandarbha (284):

Similarly in the Ramarcana-candrika it is stated:

In other words, the chanting of the Hare Krsna maha-mantra is so powerful that it does not depend on official initiation, but if one is initiated and engages in pancaratra-vidhi (Deity worship), his Krsna consciousness will awaken very soon, and his identification with the material world will be vanquished. The more one is freed from material identification, the more one can realize that the spirit soul is qualitatively as good as the Supreme Soul. At such a time, when one is situated on the absolute platform, he can understand that the holy name of the Lord and the Lord Himself are identical. At that stage of realization, the holy name of the Lord, the Hare Krsna mantra, cannot be identified with any material sound. If one accepts the Hare Krsna maha-mantra as a material vibration, he falls down. One should worship and chant the holy name of the Lord by accepting it as the Lord Himself. One should therefore be initiated properly according to revealed scriptures under the direction of a bona fide spiritual master. Although chanting the holy name is good for both the conditioned and liberated soul, it is especially beneficial to the conditioned soul because by chanting it one is liberated. When a person who chants the holy name is liberated, he attains the ultimate perfection by returning home, back to Godhead. In the words of Sri Caitanya-caritamrta (Adi 7.73):

"Simply by chanting the holy name of Krsna one can obtain freedom from material existence. Indeed, simply by chanting the Hare Krsna mantra one will be able to see the lotus feet of the Lord."

According to Caitanya-caritamrta (Madhya 17.134):

"With these material senses, one cannot understand the transcendental holy name of the Lord or His form, activities and pastimes. However, when one actually engages in devotional service, utilizing the tongue, the Lord is revealed."