A pure devotee is he who loves Krishna, without any material desire. People are generally engaged in karma. Karma means work and get the result and enjoy life. And jnana, jnana means speculating process to understand the Absolute Truth. So one who does not indulge in speculating habit, neither tries to gain something by his work, but simply engages himself in the service of the Lord, he is called a pure devotee. Such pure devotees are very rare. But by the Grace of Krishna, practically all the devotees and disciples who have kindly joined me, they are, their symptoms are pure devotees. Even if they have got some ulterior desire, that will be removed very soon, because they have taken to the pure process of Krishna Consciousness.
From Srila Prabhupada’s letter to Janaki — 13 October, 1968