By disciplic succession it is meant a line of spiritual teachers that has transmitted the transcendental knowledge until the present day in an unaltered form starting from Lord Krishna.
A spiritual master must be in an authorized disciplic succession to be bonafide. It is not possible for someone to be a bonafide spiritual master in a line of spiritual authority that does not come from Krishna.
The principle is that Vaishnava teachings should be passed on, unchanged, from guru to disciple.
In sanskrit., disciplic succession is also called "Parampara".
We can understand how perfect knowledge can be received from Lord Krishna himself. In the Bhagavad Gita, particularly in Chapter 4 Text 2 Lord Krishna mentions:
imam rajarsayo viduh
This supreme science was thus received through the chain of disciplic succession, and the saintly kings understood it in that way.
Spiritual knowledge descends through the disciplic succession from Krishna. The representatives of the disciplic succession carry the pure message of Krishna without changing anything. The transcendental knowledge (divya jnana) is like a river flowing from one acarya to the next. It's source is Krishna who directs it to a qualified disciple who realizes, preserves and directs it to his qualified disciple who again realizes, preserves and directs it to his successor and so on until it arrives here.
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