Srila Prabhupada mentions repeatedly in his books that the soul is an eternal servant of Krsna, but that due to contact with material nature we forget it. He also mentions that any progress in devotional service is permanent and therefore we never lose it. Even if we can’t complete our practice in this life, next life we continue from the point where we stopped.
This leads to the question: If devotional service is permanent, how were we able to forget Krsna in the first place? What is the difference between the service we do now, which is supposedly permanent, and the service we were doing before, which we somehow forgot? Considering this first point, what is the guarantee that we will not forget again, even if we go back to Godhead?
This is quite a complicated question because it deals with the topic of the fall of the jiva, which is a topic impossible to explain using material logic. Pure devotees can understand this topic perfectly, but it constantly baffles conditioned souls. However, considering that Srila Prabhupada is a pure devotee and an empowered acarya, we can start by trying to understand this topic from the way he was explaining it. Others prefer to explain this topic in different ways, and they surely have the right to do so, but I personally still prefer to accept it in the way Prabhupada explained.
So, starting from there, we can see that Srila Prabhupada consistently explains in his books that the soul is constitutionally a servant of Krsna and comes originally from a position of service to Krsna. When directly asked by his disciples on this topic, he answered (in a famous letter to Australian devotees in 1972) that the fall of the soul in the material world is just like a dream, where one temporarily forgets his original identity. However, this original identity is never lost, and as one awakens from this dream, one gradually remembers. In other points Srila Prabhupada explains that we have our original spiritual body and senses even right now, it’s just that at the moment it has become covered by different material layers, culminating with the material body and senses. When these coverings are removed, the original spiritual body manifests.
In this way, the eternal devotional service the soul performs in its original position is never lost, it is only temporarily forgotten. Somehow or other we temporarily forget about Krsna and fall into the dream of material existence. Here we become covered by the false ego, material intelligence, mind and senses, and finally the gross body. In this way, a new, temporary identity is formed, and identifying with this material identity we think we are separated from Krsna. From this identification, we forget Krsna and the way to come back to our original position is by, while identifying with this material identity, coming in contact with devotees and starting practicing devotional service.
We can see thus that our original devotional service is not lost, but it is covered by this material identity with which we start to identify. Under this false identification, we can stay for a very long time going from one body to the other, forgetful of our original position. When however we start “again” to practice devotional service under this temporary identity we can finally remember, just like when we start dreaming that we need to wake-up, and as a result we end up really waking up.
The devotional service we do now is also never lost, but it can be temporarily forgotten. Even a very serious devotee can fall into different traps and temporarily stop his service, or even temporarily forget about Krsna, however, the power of his past devotional service makes him eventually come back to the path and continue. Krsna Himself, as Paramatma helps the devotee to remember, inspiring him from within and putting him in situations where he can again get in contact with devotees.
In a sense, it’s because of the eternal service we perform eternally in the spiritual world that we can at a certain point remember Krsna and start practicing devotional service even while living in the material world. If the propensity of serving Krsna was not present inside the soul, we would never start serving Krsna, and as a result, we would just stay eternally here.
In this way, we can see that just like the devotional service we do here makes us continue serving Krsna in the future, up to the point we go back to Godhead, the eternal service we perform in our constitutional position (I intentionally use the verb in the present, since this service is eternal) is also the cause of we remembering Krsna and starting to serve Him even while living in the material world.
The guarantee that devotional service is never lost and the fact that everyone eventually goes back to Godhead should however not be taken as a license of slack in our spiritual practice and forget Krsna, since one who falls again under the spell of illusion can stay for a very long time in this situation, just like it happened when we somehow entered in contact with the material nature for the first time.
In his book Maya, The Divine Energy Of The Supreme, Bhakti Purusottama Swami tells the story Narada Muni asking Lord Vishnu to show him the power of his illusory energy. As a response to his request, the Lord invited him for a walk and while in a forest Narada Suddenly felt very thirsty. Lord Vishnu pointed to a nearby river and told Narada he could go there and drink and He would wait. Narada excused himself and promised to return quickly.
He arrived at the river and prepared to drink water with his joined pals when a girl appeared and asked him to instead come to her house, where they had some nice pure water. Narada accepted the invitation and was served very nicely by the girl and her father, who received Narada with all the respect deserved by a great Sadhu. Narada became pleased with their service attitude and asked if there was anything he could do for them and the father humbly asked him to marry his daughter and become his son-in-law. Attracted by the beauty of the girl and at the same time obliged by their service Narada agreed.
Narada spent many years involved in family affairs and had a few children with the girl, who served him as a faithful wife. However, one day there was a great storm, and the river overflowed, inundating the village. The children fell into the water and were pitifully drowning. The wife jumped in to try to save them, just to be defeated by the power of the currents and also start drowning herself. Narada jumped into the river to try to save his family, but he was also overpowered by the currents and started drowning. When he was almost dying, he somehow remembered the Lord and loudly called: Vishnu!
Suddenly he was back on the bank of the river where everything started. Immediately he remembered that he had left Lord Vishnu waiting for such a long time and hurried back to Him. Lord Vishnu smiled and asked if he now understood.
Of course, Narada is an eternally perfect soul, and he never falls into Maya like us. However, because he specifically asked, Lord Vishnu gave him a sample of how one can fall under the grip of material energy and forget Krsna for a long time. Srila Prabhupada explains that even if one takes birth in a good family in his next life there is no guarantee that he will become serious in the spiritual path since birth in a good family also entitles one to many material opportunities. We can practically see that even children of devotees are not always very serious in spiritual life.
The last point is how can we be sure that we will never fall down again into the material world after we come back? Srila Prabhupada explains that when Krsna tells in the Bhagavad-Gita that one who reaches the spiritual world never comes back again, it actually means “practically never” because there is always a theoretical possibility of the jiva falling again due to free will. The spiritual world is not a prison, and therefore if one wants he is free to leave at any time. However, in the spiritual world, the jiva has full knowledge and therefore is quite improbable that once experiencing the miseries of the material world one will ever want to return here, even though he has the freedom to do so.