Divya jnana and diksa

Diksa (Narada Pancaratra): di - divya jnana (about one's own svarupa and received mantra/s), ksa - destruction of sins

The process by which a devotee becomes attached to Krsna is described in Caitanya-caritamrta (Antya 4.192):

diksa-kale bhakta kare atma-samarpana
sei-kale krsna tare kare atma-sama

"At the time of initiation, when a devotee fully surrenders to the service of the Lord, Krsna accepts him to be as good as He Himself."

Diksa, or spiritual initiation, is explained in the Bhakti-sandarbha (283) by Srila Jiva Gosvami:

divyam jnanam yato dadyat
kuryat papasya sanksayam
tasmad dikseti sa prokta
desikais tattva-kovidaih

ato gurum pranamyaivam
sarvasvam vinivedya ca
grihniyad vaishnavam mantram
diksha-purvam vidhanatah || [HBV 2.9-10] ity agamat

divyam jnanam hy atra srimati mantre bhagavat-svarupa-jnanam, tena bhagavata sambandha-visesha-jnanam ca | yatha padmottara-khandadav ashtaksharadikam adhikritya vivritam asti ye tu sampattimanto grihasthas tesham tv arcana-marga eva mukhyah

It's clear that it refers to mantra vidya. The diksa mantras and their meanings are not found in the books but kept secret. Even though Srila Prabhupada made a recording of them, it was not to be made public. Similarly, the book about Gayatri mantra by Srila B.R. Sridhar M. is neither sold to the public nor available online.

Bhakti Gaurava Narasingha Maharaja (The Authorized Sri Caitanya- Saraswata Parampara 1.1) further explains:

divyam jnanam hy atra
srimati mantre bhagavata-svarupa-jnanam
tena bhagavata sambandha-visesha-jnanam ca.

"The term divine knowledge here refers to the knowledge of the specific form of the Lord contained in the holy syllables of the mantra and knowledge of a specific relationship with the Lord." (Bhakti-sandarbha, 283)

The anti-party then jumps to the conclusion that divyam jnanam (divine knowledge) means ekadasa-bhava (siddha-pranali), the knowledge of one's form as a gopi, dress, residence, etc. How-ever, this divine knowledge revealed by the guru at the time of giving diksha (as stated in Bhakti-sandarbha) is actually a reference to receiving proper understanding of the mantras for worship such as upanayana-samskara (brahma-gayatri), guru mantra and gayatri, gaura-mantra and gayatri, gopala-mantra, and kama-gayatri. There is no suitable support for ekadasa-bhava found in the above quoted verse. For a thorough understanding of this subject (revealing the meaning of the mantra receiving divyam jnanam) one should read the book Sri Gayatri Mantrartha Dipika published by Gosai Publishers."

Why physically present guru?

To hear the right news, the perfect knowledge, you must approach the bona fide spiritual master. Gurum eva abhigacchet. Must! It is not that "I can get knowledge at home. Why shall I go to guru? I do not find any guru." (SP, SB 1.2.15, Vrndavana, October 26, 1972)

tad-vijnanartham sa gurum eva abhigacchet
samit panih srotriyam brahma nistham

This verse (Mundaka Upanisad 1.2.12) is clear: gurum abhigacchet - approach the guru, samit panih - with firewood for yajna in one's hand. It doesn't say 'ritvik (etc.) abhigacchet'. For virtual-only approach no firewood is needed.

A warning against using mantras from other sources

pustake likhitAn mantrAn avalokya japet tu yah
sa jIvenn eva candAlo mRtaH syAno bhaviSyati

One who does japa of mantras seen as written in books becomes a candala (an outcaste) in the current life and after death becomes a dog. (SyAnkhyAyan Tantra)

pustake likhitAn mantrAn Alokya prajapanti ye
brahma hatyA samaM teSAM pAtakaM parikIrtitam

Doing japa of mantras seen as written in books is equal to the great sin of killing a brahmana. (Badabanala Tantra)

pustake likhito mantro yena sundarI japyate
na tasya jAyate siddhiH hAnir eva pade pade

One who does japa of Devi mantra written in books does not attain siddhi and even creates his own ruin on every step. (Brhad Badabanala Tantra)

This clearly refutes the ritviks's claim that the divya jnana is found in the books so the physically present guru is not needed, as their theory goes.

dIyate jnanavijnAnaM kShIyante pAparAshayaH
tena dIkSheti hi proktA prAptA chet sadgurormukhAt

That which gives jnana and vijnana and destroys sins is called diksa. It is obtained through the mouth of sad guru. (Meru Tantra)

na hi pApAbhibhUtena dIkSA kAryA tu vaiSNavI

Nor is the diksa to be given to people touched by sin. (Parama samhita 17.2)

Diksa is taken only once. (Sanatkumara samhita 97)

If taken from a non-Vaisnava, bhakti doesn't grow. (Narada Pancaratra 2.2.12)

evaM teSAM pArSadAnAM nAsti mRtyur yathA hareH
yasmin dehe labhen mantraM vaiSNavo vaiSNavAd api 23
pUrva-karmAzritaM dehaM tyaktvA sa pArSado bhavet
paJca-vaktreNa satataM tan-nAma-guNa-kIrtanam 24

Again like Hari, His Parsadas (attendants) do not die. The bodies in which the Vaisnavas take the mantra from the Vaisnavas are due to their previous karmas; hence they quit their bodies and then attain the positions of Parsadas [siddha deha]. With my five faces I [Siva] always recite the praises of the name of Sri Hari. (Narada Pancaratra 2.2.23-24)

Elaborate information about diksa is in Caitanya Caritamrta Madhya 15.108-110, p. and Kularnava tantra, listing a wide range of qualities of guru and disciple.