Who can understand the mystery of bhajan?

Only the unalloyed servants of the spiritual masters Sri Svarupa and Sri Rupa can understand the mystery of worshipping the Lord. A devotee who has firm faith in and love for the spiritual master’s lotus feet is called an unalloyed servant.

The Vedas declare:

yasya deve parā bhaktir
yathā deve tathā gurau
tasyaite kathitā hy arthāḥ
prakāśante mahātmanaḥ

“Only unto those great souls who have implicit faith in both the Lord and the spiritual master are all the imports of Vedic knowledge automatically revealed.” ~ (Śvetāśvatara Upaniṣad 6.23)