More we realize Krishna’s mercy, the more we become submissive and more we become surrendered.
Unless and until we surrender to Krishna, Krishna is not going to take our responsibility.
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.