What is the difference between a pure devotee and a mixed devotee ?
Srila Prabhupada..."The sunrays are open to everyone, but the capacities of the receptacles differ. Foolish people think that devotional service is flattering the Lord to get special mercy. Factually the pure devotees who are engaged in the transcendental loving service of the Lord are not a mercantile community. A mercantile house renders service to someone in exchange for values. The pure devotee does not render service unto the Lord for such exchange, and therefore the full mercy of the Lord is open for him. Suffering and needy men, inquisitive persons, or philosophers make temporary connections with the Lord to serve a particular purpose.
When the purpose is served, there is no more relation with the Lord. A suffering man, if he is pious at all, prays to the Lord for his recovery. But as soon as the recovery is over, in most cases the suffering man no longer cares to keep any connection with the Lord. The mercy of the Lord is open for him, but he is reluctant to receive it. That is the difference between a pure devotee and a mixed devotee. Those who are completely against the service of the Lord are considered to be in abject darkness, those who ask for the Lord’s favor only at the time of necessity are partial recipients of the mercy of the Lord, and those who are cent-percent engaged in the service of the Lord are full recipients of the mercy of the Lord. Such partiality in receiving the Lord’s mercy is relative to the recipient, and it is not due to the partiality of the all-merciful Lord."
Srila Prabhupada's book Teachings of Queen Kunti : TQK 12: Bewildering Pastimes