When one serves a Vaisnava unknowingly, one still gets the good result, and if one unknowingly insults a Vaisnava, one suffers the bad result. A Vaisnava is especially favored by the Supreme Personality of Godhead. Pleasing him or displeasing him directly affects the pleasure and displeasure of the Supreme Lord. Srila Visvanatha Cakravarti Thakura, in his eight stanzas of prayer to the spiritual master, has sung, yasya prasadad bhagavat-prasadah: by pleasing the spiritual master, who is a pure Vaisnava, one pleases the Personality of Godhead, but if one displeases the spiritual master one does not know where he is going.

Srila Prabhupada – SB 4.8.23  purport