Such determination is the first qualification. As confirmed by Rupa Gosvami (Upadesamrta 3): utsahan niscayad dhairyat. One must first have firm determination, firm faith. When one engages in devotional service, he must maintain this firm determination. Then Krsna will be pleased with his service. The spiritual master can show the path of devotional service. If the disciple follows the principles rigidly and undeviatingly, he will certainly receive the mercy of Krsna. This is confirmed by the sastras.