Because the material world is full of obstructions to advancement in Krishna consciousness, there may appear to be many impediments, yet Krishna, the Supreme Personality of Godhead, declares in Bhagavad-gita (9.31), kaunteya pratijanihi na me bhaktah pranasyati: once one has taken shelter at the lotus feet of the Lord, he cannot be lost. The devotional path is so auspicious that a devotee cannot be lost under any circumstances. This is described in the Srimad Bhagavad-gita by the Lord Himself Partha naiveha namutra vinasas tasya vidyate: “My dear Arjuna, for a devotee there is no question of being lost, either in this life or in the next.” (Bg. 6.40) In Bhagavad-gita (6.43) the Lord clearly explains how this is so.
tatra tam buddhi-samyogam labhate paurva-dehikam
yatate ca tato bhuyah samsiddhau kuru-nandana
By the order of the Lord, a perfect devotee sometimes comes to this material world like an ordinary human being. Because of his previous practice, such a perfect devotee naturally becomes attached to devotional service, apparently without cause. Despite all kinds of impediments due to surrounding circumstances, he automatically perseveres in devotional service and gradually advances until he once again becomes perfect. Bilvamangala Thakura had been an advanced devotee in his previous life, but in his next life he became greatly fallen and was attached to a prostitute. Suddenly, however, his entire behaviour was changed by the words of the very prostitute who had so much attracted him, and he became a great devotee. In the lives of exalted devotees, there are many such instances, proving that once one has taken to the shelter of the lotus feet of the Lord, he cannot be lost (kaunteya pratijanihi na me bhaktah pranasyati).
Source: A.C. Bhaktivedanta Swami Prabhupada (2014 edition), “Srimad Bhagavatam”, Fifth Canto, Chapter 01 – Text 05