Srila Prabhupada...."The spiritual master is addressed as “Prabhupāda” because he is a completely self-realized soul. The word pāda means “position,” and Prabhupāda indicates that he is given the position of prabhu, or the Supreme Personality of Godhead, for he acts on behalf of the Supreme Personality of Godhead. Unless one is a prabhu, or controller of the senses, he cannot act as spiritual master, who is authorized by the supreme prabhu, or Lord Kṛṣṇa."
Srila Prabhupada...."The specific qualification for becoming the representative of the Lord is to be unaffected by the material modes of nature. The highest qualification of a person in the material world is to be a brāhmaṇa. But since a brāhmaṇa is in the mode of goodness, to be a brāhmaṇa is not sufficient for becoming a representative of the Lord. One has to transcend the mode of goodness also and be situated in unalloyed goodness, unaffected by any of the qualities of material nature. This stage of transcendental qualification is called śuddha-sattva, or vasudeva, and in this stage the science of God can be realized. As the Lord is not affected by the modes of material nature, so a pure devotee of the Lord is also not affected by the modes of nature. That is the primary qualification for being one with the Lord. A person who is able to attain this transcendental qualification is called jīvan-mukta, or liberated, even though he is apparently in material conditions. This liberation is achieved by one who constantly engages in the transcendental loving service of the Lord"