Srila Prabhupada

Bhagavad-gita 4.9
One who knows the transcendental nature of My appearance and activities does not, upon leaving the body, take his birth again in this material world, but attains My eternal abode, O Arjuna.

(For many devotees they think after reading the above verse many times and Krishna book as well – Yes. I understand Krishna and His activities, so that means I will be going back to Godhead after death." Here is the reality explained more fully below.)

Srimad-Bhagavatam 1.18.3 purport
…From Bhagavad-gītā we understand the quality of the Lord's energy that is superior to the material quality of energy, and by the grace of a bona fide spiritual master like Śukadeva Gosvāmī, it is quite possible to know everything of the superior energy of the Lord by which the Lord manifests His eternal name, quality, pastimes, paraphernalia and variegatedness. Unless one thoroughly understands this superior or eternal energy of the Lord, it is not possible to leave the material energy, however one may theoretically speculate on the true nature of the Absolute Truth.

Conversation – November 2, 1975, Nairobi

Harikesha: I’m curious about the destination of a neophyte devotee. If a neophyte devotee is with determination endeavoring for purification but he were to meet with death as he is still influenced by the lower modes, although he is seriously trying, then does he take another birth or does he go to Krishna?

Prabhupada: No, he has to take another birth. If he is not completely purified, he has to suffer another birth. Nobody is allowed to enter into the spiritual unless he is cent percent pure. No allowance. 

Harikesha: So if a devotee dies and remembers Krishna, although he is not perfect…

Prabhupada: Unless he is perfect, he cannot remember Krishna. That is not possible. That is not possible. That is theory only. He must be perfect. Somehow or other, he fallen, so Krishna gives him the chance. That is special concession for devotee. Some way or other, you become devotee. Even if you cannot finish the whole job, if you fall down, still, there is guarantee that you get your birth in a very good society. That is the prerogative.