Impersonalists cannot be purified, for they do not offer personal prayers to the Supreme Personality of Godhead. Even though they sometimes offer prayers, the prayers are not directed toward the Supreme Person. Impersonalists sometimes show their incomplete knowledge by addressing the Lord as being nameless.
They always offer prayers indirectly, saying, "You are this, You are that," but they do not know to whom they are praying. A devotee, however, always offers personal prayers.
A devotee says, govindam adi-purusam tam aharm bhajami, "I offer my respectful obeisances unto Govinda, unto Krsna." That is the way to offer prayers. If one continues to offer such personal prayers to the Supreme Personality of Godhead, he is eligible to become a pure devotee and return home, back to Godhead.