As Srila Prabhupada said in a famous lecture, there are no barriers in terms of how fast one can advance in Krsna Consciousness. One who is exceptionally sincere can become pure even in a single moment. One just has to chant the holy names one time with a pure heart, that’s all that it takes.
However, most of us are not ready for such a quick evolution and thus we often have to practice for several lives to be able to attain perfection. On his Jaiva Dharma, Srila Bhaktivinoda Thakura tells stories of several personalities on different stages of spiritual evolution: some still on a sinful platform, others on a pious platform, and a few on a truly spiritual platform. Souls can pass through all these different platforms over a sequence of lives, going from some sinful offender, like Vani-Madhava, all the way to someone who starts from an almost pure platform, like Vaishnava Dasa.
For most of us, that’s the progression: we start from a sinful platform, and the first step is to accept the rules and regulations of spiritual life and gradually, through practice, purify our hearts from the previous contamination. That’s the stage of a Kanista-Adhikari devotee, who struggles to follow the basic principles of spiritual life.
As one matures on the first platform and his heart becomes purified, he ascends to a pious platform, where he doesn’t have so much problem with grossly sinful habits, but he still has a problem with his ego: a desire for being respected, for becoming an important figure, to convince others of his opinions, to have a social position, etc. At this stage, the natural occupation for one on the spiritual path is to become a preacher and use this tendency of conquest to spread Krsna Consciousness. This is the stage of a Madhyama-Adhikari, who although not completely perfect yet, can advance further by working for Krsna.
As one progresses in this path, the last vestiges of contamination in the heart are destroyed, and he becomes completely humble. Is at this stage that one can finally start truly developing his relationship with Krsna. The general tendency at this stage is that one becomes a humble devotee living in simple conditions and using all his energy to serve, or sometimes a reclused Babaji. This is the uttama-adhikari stage, where one can purely serve Krsna without any false prestige. This is the platform of one who is preparing to go back to Godhead.
We can see that some Vaishnavas go through these three stages in a single life, going quickly through the Kanista-adhikari stage, then becoming a fired-up preacher and eventually reaching this stage of humility and renunciation later in his life. One who is sincere can complete the path in just one life, but most of us generally need several lives to attain the perfect stage.
However, there is a stage that goes even further. Some very special devotees, after attaining the final platform of humility, adjust their mentality back to the Madhyama-Adhikari stage, and thus, acting according to the precepts of the scriptures, become empowered preachers, who can spread Krsna Consciousness far and wide, and change people’s hearts by their purity and personal example. At this stage, one can become an empowered acarya and send thousands of disciples back to Godhead.
Such exalted devotees are already on a liberated platform, their consciousness focused on Krsna, but they remain in this material world to share their experience with us. For them, there is no need to go back to Godhead, because they already attained the ultimate goal. On this platform, they just want to serve Krsna. If Krsna wants, they can spend more time in this material world to help other souls, or if Krsna wants they can go to the spiritual world and serve Him there.
As Srila Prabhupada once explained, some devotees (the madhyama-Adhikaris) like to preach and some (the Uttama-Adhikaris) like to chant. However, some Uttama-Adhikaris also preach, and these are the best of all.
Srila Prabhupada describes the characteristics of devotees who have attained such an exalted platform in his purport on the 5th verse of Sri Updesamrta:
“When a person realizes himself to be an eternal servitor of Kṛṣṇa, he loses interest in everything but Kṛṣṇa’s service. Always thinking of Kṛṣṇa, devising means by which to spread the holy name of Kṛṣṇa, he understands that his only business is in spreading the Kṛṣṇa consciousness movement all over the world. Such a person is to be recognized as an uttama-adhikārī, and his association should be immediately accepted according to the six processes (dadāti pratigṛhṇāti, etc.). Indeed, the advanced uttama-adhikārī Vaiṣṇava devotee should be accepted as a spiritual master.”
He however also alerts us to the dangers of artificially imitating such an exalted platform. We can see that this also happened at certain points:
“However, one should not imitate the behavior of an advanced devotee or mahā-bhāgavata without being self-realized, for by such imitation one will eventually become degraded.”