Please explain in detail what Sri Krishna wants to say in the verse
18.66 of Srimad Bhagwad Gita :
१८-६६|| सर्वधर्मान्परित्यज्य मामेकं शरणं व्रज | अहं त्वा सर्वपापेभ्यो मोक्षयिष्यामि मा शुचः
mām ekaṁ śaraṇaṁ vraja
ahaṁ tvāṁ sarva-pāpebhyo
mokṣayiṣyāmi mā śucaḥ
Abandon all varieties of religion and just surrender unto Me. I shall deliver you from all sinful reactions. Do not fear.
Please refer: http://www.vedabase.com/en/bg/18/66 for full explanation by Srila Prabhupada.
This verse should better be interpreted with context of other verses. The independent analysis/translation may lead to entirely different interpretation, as it was done in Indian movie "Dharam Sankat Mein". Let's see in bits & pieces.
BG 18.66 - Abandon all Dharma-s [and] take surrender [only] under "Me". "I" will liberate you from all sins. Don't repent.
Here, "I" & "Me" should be interpreted as the ultimate One, rather than Krishna's personal form as discussed in this answer.
Krishna is a rare philosopher, who had represented the ultimate identity with "I". This "I" is the liberator or the liberation.
Now if we surrender to "I", then it simply liberates us from whatever sin we have incurred! That actually contradicts with other Vedic literature, which stresses on Karma & bearing its good/bad fruits. Doesn't it challenge common sense?
Yes, it does. But it's still true.
Assume 2 planes:
In plane-1, we have a perception of good Karma & bad Karma, which affects us. However in plane-2, these sins (bad deeds) & virtues (good deeds) are not accepted, rather they are treated as illusions.
BG 5.15 - The Omnipresent neither accepts anybody's sin nor even virtue. Knowledge remains covered by ignorance. Thus the creatures become deluded.
Krishna asks "not to repent/grieve", in the end of the verse.
We may think, provided that we are still in the material world, aren't we still under the spell of Karma? Won't good/bad Karma affect our past/present/future lives? If we have committed a sin then, shouldn't we worry about its repentance or prAyaschita.
Such repentance depends on the ignorance. Suppose, if I believe that, I have committed a sin, then I may want to do repentance. According to Gita, that's ignorance. In reality, no Karma happens by oneself. Only 3 modes of material nature (Sattva, Rajas, Tamas), makes all the Karma possible. Why should one repent (do prAyaschita), when the consciousness of oneself just resides as a witness or a consumer.
BG 3.27 - Actions are performed by [three] modes of [material] nature. [Purusha] Deluded by egoism believes as, "I am the do-er".
The most tricky part. First of all, we should understand that Dharma is Not religion. A person can have no religion, but every entity has its own Dharma in material world. The "I" also has its own Dharma, which is supreme.
The closest single word meaning of Dharma is Tendency.
The 3 modes of material nature as discussed above, are related to Dharma. BG 18.30-18.32:
Everyone has these 3 modes present within them except the "I". "I" is beyond these 3 modes. So, Dharma is a strong & easy tool for SAtvika people towards liberation. But there is no specific requirement that SAtvika people will get quicker liberation/Moksha compared to RAjasika or TAmasika.
For example, ShishupAla who had spent his life in ravaging innocent people got liberation as discussed in SabhA parva. Similarly Kansa, who killed innocent infants by throwing them to a wall, also got Moksha. Both of them showed Tamas qualities. There might be back-story explanations of these 2 (e.g. Jaya-Vijaya), but this is a neutral point of view. Similar can be said about RAvana as well, who was RAjasika (a Rakshasa).
Arjuna was born with Daivi Sampada (Divine Quality) as he was predominantly SAtvika. As noted in Shankara's commentary, the Gita was said with respect to Arjuna representing the humanity.
Arjuna is not alified for steadfastness in Knowledge through monasticism in the primary sense. Still, the Gita being meant for mankind as a whole, monasticism is spoken of here by accepting Arjuna as a representative man.
Now, Sattva or "following of Dharma" also refrains a person from liberation. It holds a Purusha within material nature by knowledge & happiness! This is a rare caveat found in Gita and this is the primary reason for abandoning all Dharma to be able to merge with the supreme/"I".
BG 14.5,6,7,9 — O might armed, the modes of Sattva, Rajas & Tamas generated from Prakruti, binds the embodied Indivisible (Atman) into (subtle) body. — O sinless, Due to purity, the Sattva is illuminating & distortionless. It binds with happiness & with knowledge. — O son of Kunti, know that Rajas is generated from self-desire & thirst (to achieve). That binds the embodied (jeeva) with Karma. — O Bharata, know Tamas to be born from ignorance. It binds all bewildered embodied (jeeva) with enjoyment, laziness, sleep.
Though Krishna followed Dharma all his life, he never refrained from breaking stereotype whenever needed. As discussed in this answer, he didn't allow Sattva to overcome him. Rather he remained beyond 3 modes.
Still, sattva is foundation for any good society and many people follow it for smooth liberation. Treat Dharma as a boat in the river like life. Once you reach the other side of the river, where the "I" abodes, leave the boat!