You should know that the Hare Krishna Mantra has an equal effect upon all devotees. Just like the sunshine has an equal effect on everyone, but when it is covered, the sunshine has a different effect. Similarly, the influence of the Hare Krishna Mantra becomes manifest when one is no longer covered by the ten offenses to chanting. The more we become free from the ten offenses, the more the effect of chanting becomes manifested through us. Everyone can become a great devotee, being freed from the offenses 100%, simply by one's determination and effort.
Letter to Arundhati, 16 June, 1969