Vivekachudamani 17

vivekino viraktasya samadigunasalinah | 
mumuksoreva hi brahmajijñasayogyata mata || 
(Sankaracarya's Vivekachudamani 17) 

He alone is considered qualified to enquire after the supreme Reality, who has discrimination, detachment, qualities of calmness etc., and a burning desire for liberation. 

As it is stated in the Srimad-Bhagavatam, 

vasudeve bhagavati 
bhakti-yogah prayojitah 
janayaty asu vairagyam 
jnanam ca yad ahaitukam 
[SB 1.2.7] 

"If somebody engages himself in the devotional service of Vasudeva, then very quickly this detachment develops. And knowledge." Knowledge also. Without knowledge, there cannot be detachment. Without knowledge, one cannot be detached. And what is that knowledge? The knowledge is that "I am not this matter; I am spirit soul." So... But this knowledge is... Although it is very easy thing to say, that "I am not this body, but I am spirit soul," but actually to have perfect knowledge, that is a great job. It is not very easy. For getting that supreme knowledge so many, I mean to say, transcendentalists, they were trying life after life, just to get detached. But the easiest process is that if one is engaged in the devotional service. That is the formula given in Srimad-Bhagavatam. Vasudeve bhagavati. Vasudeve bhagavati, "in the Personality of Godhead, Krsna." Vasudeva is Krsna. Vasudeve bhagavati bhakti-yogah prayojitah [SB 1.2.7]. Bhakti-yoga means devotional service. If it is applied, if we apply our devotional service unto the Supreme Lord Sri Krsna, then the result will be that janayaty asu vairagyam, very soon you'll be detached from this material attraction. Very soon. And jnanam ca yad ahaitukam. And you'll gain also knowledge. You do not know how you have received knowledge. That is the magic. That is the magic. Because how you'll get knowledge? That is also stated in the Bhagavad-gita. 

Just like Lord Buddha's teaching. Lord Buddha's teachings is... That is also detachment from matter, nirvana. Nirvana. But he does not speak anything about the spirit soul. Because the position in which he was speaking, that position, for the human, humanity, was not suitable for understanding what is the constitution of spirit, therefore he did not say anything about spirit. He simply preached nonviolence. So far our body is concerned, he stressed on the point that we should be nonviolent. We should not be killing animals anymore. That was his preaching. Similarly, Sankara... A little more than Buddha. He said, "No, no. The matter is not all. The spirit is the real thing. The matter is false." Brahma satyam jagan mithya. Now, he did not say about the activities of spiritual life. He simply gave hint that there, that matter is false. The matter is generated by spirit. Spirit is the real, principal thing. Just like Buddha did not say anything about spirit. He simply wanted that detachment of, from matter. But detachment from matter... Then where is my stand? Where is my stand? If I leave this room, I must have another room to stay. So that is the position of Buddha. He did not say about the spirit. But Sankara, Sankara said, "No, the matter is our false position. Spirit is real position." But he did not say anything, what are the activities of the spiritual life. Then Sri Ramanujacarya came. He described the actual position of spiritual life. These are gradual development. Your, I mean to say, Lord Jesus, also, Lord Jesus Christ, he also gave hints of spiritual life, the kingdom of God. So when we speak of kingdom of God, a kingdom, vacant, cannot be. Kingdom means there must be activities. Otherwise, what is the meaning of the kingdom? So, of course, he did not give any detailed account of the kingdom of God, but he gave a hint.