In the preliminary phase of spiritual life there are different kinds of austerities, penances and similar processes for attaining self-realization. However, even if an executor of these processes is without any material desire, he still cannot achieve devotional service. And aspiring by oneself alone to achieve devotional service is also not very hopeful because Krsna does not agree to award devotional service to merely anyone. Krsna can easily offer a person material happiness or even liberation, but He does not agree very easily to award a person engagement in His devotional service. Devotional service can in fact be attained only through the mercy of a pure devotee. In the Caitanya-caritamrta Madhya 19.151 it is said, "By the mercy of the spiritual master who is a pure devotee and by the mercy of Krsna one can achieve the platform of devotional service. There is no other way."
The rarity of devotional service is also confirmed in the Tantra-sastra, where Lord Siva says to Sati, "My dear Sati, if one is a very fine philosopher, analyzing the different processes of knowledge, he can achieve liberation from the material entanglement. By performance of the ritualistic sacrifices recommended in the Vedas one can be elevated to the platform of pious activities and thereby enjoy the material comforts of life to the fullest extent. But all such endeavors can hardly offer anyone devotional service to the Lord, not even if one tries for it by such processes for many, many thousands of births."
In the Srimad-Bhagavatam it is also confirmed by Prahlada Maharaj that merely by personal efforts or by the instructions of higher authorities one cannot attain to the stage of devotional service. One must become blessed by the dust of the lotus feet of a pure devotee who is completely freed from the contamination of material desires.
In the Fifth Canto of Srimad-Bhagavatam, 6th Chapter, 18th verse, Narada also says to Yudhisthira, "My dear King, it is Lord Krsna, known as Mukunda, who is the eternal protector of the Pandavas and the Yadus. He is also your spiritual master and instructor in every respect. He is the only worshipable God for you. He is very dear and affectionate, and He is the director of all your activities, both individual and familial. And what's more, He sometimes carries out your orders as if He were your messenger! My dear King, how very fortunate you are, because for others all these favors given to you by the Supreme Lord would not even be dreamt of." The purport to this verse is that the Lord easily offers liberation, but He rarely agrees to offer a soul devotional service because by devotional service the Lord Himself becomes purchased by the devotee.
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