Śrīla Jīva Gosvāmī explains dīkṣā in his Bhakti-sandarbha (283):

divyaṁ jñānaṁ yato dadyāt kuryāt pāpasya saṅkṣayam
tasmāt dīkṣeti sā proktā deśikais tattva-kovidaiḥ

“Dīkṣā is the process by which one can awaken his transcendental knowledge and vanquish all reactions caused by sinful activity. A person expert in the study of the revealed scriptures knows this process as dīkṣā.” The regulative principles of dīkṣā are explained in the Hari-bhakti-vilāsa (2.3-4) and the Bhakti-sandarbha (283). As stated:

dvijānām anupetānāṁ svakarmādhyayanādiṣu
yathādhikāro nāstīha syāc copanayanād anu
tathātrādīkṣitānāṁ tu mantra-devārcanādiṣu
nādhikāro ’sty ataḥ kuryād ātmānaṁ śiva-saṁstutam

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

According to the Vaiṣṇava regulative principles, one must be initiated as a brāhmaṇa. The Hari-bhakti-vilāsa (2.6) quotes the following injunction from the Viṣṇu-yāmala:

adīkṣitasya vāmoru kṛtaṁ sarvaṁ nirarthakam
paśu-yonim avāpnoti dīkṣā-virahito janaḥ

“ ‘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.’ ”

Hari-bhakti-vilāsa (2.10) further quotes:

ato guruṁ praṇamyaivaṁ sarva-svaṁ vinivedya ca
gṛhṇīyād vaiṣṇavaṁ mantraṁ dīkṣā-pūrvaṁ vidhānataḥ

“ ‘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 Vaiṣṇava initiation from him.’ ”

The Bhakti-sandarbha (298) gives the following quotation from the Tattva-sāgara:

yathā kāñcanatāṁ yāti kāṁsyaṁ rasa-vidhānataḥ
tathā dīkṣā-vidhānena dvijatvaṁ jāyate nṛṇām

“ ‘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 brāhmaṇa.’ ”

The Hari-bhakti-vilāsa (17.11-12), in discussing the puraścaryā process, quotes the following verses from the Agastya-saṁhitā:

pūjā traikālikī nityaṁ japas tarpaṇam eva ca
homo brāhmaṇa-bhuktiś ca puraścaraṇam ucyate

guror labdhasya mantrasya prasādena yathā-vidhi
pañcāṅgopāsanā-siddhyai puraś caitad vidhīyate

“ ‘In the morning, afternoon and evening one should worship the Deity, chant the Hare Kṛṣṇa mantra, offer oblations, perform a fire sacrifice and feed the brāhmaṇas. These five activities constitute puraścaryā. To attain full success when taking initiation from the spiritual master, one should first perform these puraścaryā processes.’ ”

The word puraḥ means “before,” and caryā means “activities.” Due to the necessity of these activities, we do not immediately initiate disciples in the International Society for Krishna Consciousness. For six months, a candidate for initiation must first attend ārati and classes in the śāstras, practice the regulative principles and associate with other devotees. When one is actually advanced in the puraścaryā-vidhi, he is recommended by the local temple president for initiation. It is not that anyone can be suddenly initiated without meeting the requirements. When one is further advanced by chanting the Hare Kṛṣṇa mantra sixteen rounds daily, following the regulative principles and attending classes, he receives the sacred thread (brahminical recognition) after the second six months.

In the Hari-bhakti-vilāsa (17.4-5, 7) it is stated:

vinā yena na siddhaḥ syān mantro varṣa-śatair api
kṛtena yena labhate sādhako vāñchitaṁ phalam

puraścaraṇa-sampanno mantro hi phala-dhāyakaḥ
ataḥ puraṣkriyāṁ kuryāt mantra-vit siddhi-kaṅkṣayā

puraṣkriyā hi mantrāṇāṁ pradhānaṁ vīryam ucyate
vīrya-hīno yathā dehī sarva-karmasu na kṣamaḥ
puraścaraṇa-hīno hi tathā mantraḥ prakīrtitaḥ

“Without performing the puraścaryā activities, one cannot become perfect even by chanting this mantra for hundreds of years. But one who has undergone the puraścaryā-vidhi process can attain success very easily. If one wishes to perfect his initiation, he must first undergo the puraścaryā activities. The puraścaryā 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 puraścaryā-vidhi, no mantra can be perfected.”

In his Bhakti-sandarbha (283-84), Śrīla Jīva Gosvāmī describes the importance of Deity worship and initiation (dīkṣā) as follows:

yadyapi śrī-bhāgavata-mate pañcarātrādi-vat arcana-mārgasya āvaśyakatvaṁ nāsti, tad vināpi śaraṇāpattyādīnām ekatareṇāpi puruṣārtha-siddher abhihitatvāt, tathāpi śrī-nāradādi-vartmānusaradbhiḥ śrī-bhagavatā saha sambandha-viśeṣaṁ dīkṣā-vidhānena śrī-guru-caraṇa-sampāditaṁ cikīrṣadbhiḥ kṛtāyāṁ dīkṣāyām arcanam avaśyaṁ kriyetaiva.

yadyapi svarūpato nāsti, tathāpi prāyaḥ svabhāvato dehādi-sambandhena kadarya-śīlānāṁ vikṣipta-cittānāṁ janānāṁ tat-tat-saṅkocī-karaṇāya śrīmad-ṛṣi-prabhṛtibhir atrārcana-mārge kvacit kvacit kācit kācin maryādā sthāpitāsti.

“It is Śrīmad-Bhāgavatam’s opinion that the process of Deity worship is not actually necessary, just as the specific prescriptions of the Pañcarātra and other scriptures do not have to be followed. The Bhāgavatam enjoins that even without practicing Deity worship one can achieve the complete success of human life by any of the other devotional processes, such as simply offering oneself at the Lord’s feet for His protection. Nonetheless, Vaiṣṇavas following the path of Śrī Nārada and his successors endeavor to establish a personal relationship with the Lord by receiving the grace of a bona fide spiritual master through initiation, and in this tradition the devotees are obliged at the time of initiation to begin engaging in Deity worship.

“Although Deity worship is not essential, the material conditioning of most candidates for devotional service requires that they engage in this activity. When we consider their bodily and mental conditions, we find that the character of such candidates is impure and their minds are agitated. Therefore, to rectify this material conditioning the great sage Nārada and others have at different times recommended various kinds of regulations for Deity worship.”

Similarly, the Rāmārcana-candrikā states:

vinaiva dīkṣāṁ viprendra puraścaryāṁ vinaiva hi
vinaiva nyāsa-vidhinā japa-mātreṇa siddhi-dā

“O best of the brāhmaṇas, even without initiation, preliminary purification or acceptance of the renounced order, one can attain perfection in devotional service simply by chanting the Lord’s holy name.”

In other words, the chanting of the Hare Kṛṣṇa mahā-mantra is so powerful that it does not depend on official initiation, but if one is initiated and engages in pañcarātra-vidhi (Deity worship), his Kṛṣṇa 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, one 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 Kṛṣṇa mantra, cannot be identified with any material sound. If one accepts the Hare Kṛṣṇa mahā-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 Śrī Caitanya-caritāmṛta (Ādi 7.73):

kṛṣṇa-mantra haite habe saṁsāra-mocana
kṛṣṇa-nāma haite pābe kṛṣṇera caraṇa

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

The offenseless chanting of the holy name does not depend on the initiation process. Although initiation may depend on puraścaryā or puraścaraṇa, the actual chanting of the holy name does not depend on puraścaryā-vidhi, or the regulative principles. If one chants the holy name even once without committing an offense, he attains all success. During the chanting of the holy name, the tongue must work. Simply by chanting the holy name, one is immediately delivered. The tongue is sevonmukha-jihvā — it is controlled by service. One whose tongue is engaged in tasting material things and also talking about them cannot use the tongue for absolute realization.

ataḥ śrī-kṛṣṇa-nāmādi na bhaved grāhyam indriyaiḥ
sevonmukhe hi jihvādau svayam eva sphuraty adaḥ

“With the material senses one cannot understand the transcendental holy name of the Lord or His form, activities and pastimes. But when one actually engages in devotional service, utilizing the tongue, the Lord is revealed.” As it is said in the Caitanya-caritāmṛta (Madhya 17.134):

ataeva kṛṣṇera ‘nāma’, ‘deha’, ‘vilāsa’,
prākṛtendriya-grāhya nahe, haya sva-prakāśa

“The holy name of Kṛṣṇa, His body and His pastimes cannot be understood by the blunt material senses. They are manifested independently.”