By practice or by becoming sinless we will not become a pure devotee but by surrendering to Krishna we will become a pure devotee.
It is not that the moment one comes to the platform of devotion, he immediately achieves his perfection. Gradually by performing devotional service he comes to the platform of perfection.
By only reading scriptures, without devotee association one becomes an impersonalist.
The biggest hindrance to our spiritual progress is Vaishnava Aparadh.