When you want to buy something, you have to give something in return. Therefore, if you want to buy pure devotional service, you have to purchase it with your faith. To the degree that you place your faith, to that degree you will get some return for it. Advancement in devotional service is simply advancement to the degree of our faith, and that faith will be tested along the way.
LFD Vol 3,Cultivating Krishna Conciousness Pg 169, 2nd last paragraph.