Your final question is how can you become a pure devotee, and the answer to this is given in Bhagavad-gita. Krishna instructs Arjuna that the mahatma, the pure devotee, is he who has fully surrendered himself unto the spiritual nature. And how can we tell if one is completely surrendered unto the spiritual nature? He is always fully engaged in devotional service to Krishna. So this is the process of becoming a pure devotee. We have to engage all of our words, thoughts, and activities in Krishna Consciousness under the direction of the Spiritual Master. Therefore, at every moment you should be busy in some way in serving Krishna and His pure devotees, and automatically you will become purified in Krishna Consciousness. Krishna is always willing to shower His mercy upon His devotees, so we have simply to become receptive to this mercy by devotional service, and then the path is very easy.

Srila Prabhupada — Letter, June 16, 1969