From – The forty rules of love and life by Shams Tabrizi

35. In this world, it is not similarities or regularities that take us a step forward, but blunt opposites. And all the opposites in the universe are present within each and every one of us. Therefore the believer needs to meet the unbeliever residing within. And the unbeliever should get to know the silent faithful in him. Until the day one reaches the stage of being the perfect human being, faith is a gradual process and one that necessitates its seeming opposite: disbelief.