BG Chap 9 - The devotee quickly becomes righteous
How can a sinful person be considered saintly, when the scriptures condemn him? The Kaṭha Upaniṣad (2.24) says that one who has not given up his sinful acts and not in control of his senses cannot attain the Lord even thru knowledge.
But this is applicable only to persons whose very nature is sinful. Kṛṣṇa says that one who worships Him exclusively, very quickly becomes righteous and becomes freed from contamination. The devotee is purified of his incidental sinful acts by Kṛṣṇa, who holds the devotee in His mind. The devotee quickly becomes steady in proper conduct. Continually repenting his previous acts, he attains complete cessation of all those sinful acts. Even if the devotee develops righteousness only in the future, still the righteousness exists right now in a subtle form. After taking medicine, though the effects of fever remain for some time, it is not considered serious. Due to bhakti, the devotee has a pure heart even during the stage of having tendency to commit sins.