I explain the concept of how spiritual progress is not lost, first by reading the first Bhagavad-gita verse to describe this, which is found in Chapter 2, verse 40. I then present more what Krishna explains in Chapter 6 of the Gita and give examples from the Srimad Bhagavatam to boost.
Here’s verse 2.40:
pratyavāyo na vidyate
sv-alpam apy asya dharmasya
trāyate mahato bhayāt
“Effort does not fail here; there is no falling back. Even very little of this dharma delivers one from great danger.”