After Initiation in a Gaudiya Vaishnava Lineage, the very 1st Mantra (Vrindavan) a disciple gets from his Spiritual Master is of Guru Manjari- “Radha Priyaye Dhimahi” i.e., my Guru Manjari is the dearmost friend (Sakhi) of Srimati Radharani.

Till one understands this and has firm belief in this (that very dearmost sakhi of Radha’rani is in front of me), one cannot attain siddhi of this mantra..and without this,one can NEVER EVER attain siddhi of other mantras.