The more you forget about nonsense material enjoyment, you must know you are advancing in Krishna Consciousness. You haven't got to ask anybody how you are making progress, you will realize yourself by this test. Just like a person taking lunch, he will feel satisfied of hunger and strength, himself. Similarly, the more you serve Krishna, you will forget material hankerings and get spiritual strength. This is the test.
(Letter to Sacisuta---17th. June, 1978).

Our standard of purity must be kept, otherwise there will be loss of spiritual strength, and subsequent fall down. Everyone of you should be thoroughly convinced of the power of the Hare Krsna mantra to protect you in all circumstances and chant accordingly at all times without offense.
(Letter to Damodara---10th. January, 1971).

The standard of purity which I have introduced in the Krsna Consciousness movement will give you the real spiritual strength needed to preach.
(Letter to Tirthapada---13th. November, 1970).

Spiritually strong means to have good faith in Kṛṣṇa and guru. That is spiritual strength. Therefore Baladeva means guru, spiritual strength.
(Lecture on Srimad Bhagavatam---1:5:22---3rd. August, 1974).

Yes. Spiritual strength means sincerity and sincerity means spiritual strength.
(Morning Walk---14th. May, 1975).

So far we are concerned, every one of us must chant the beads according to the vow and follow the regulative principles in all departments of our activities, and this will give us spiritual strength to convince the audience about our aims and object.
(Letter to Tamala Krsna---20th. February, 1970).

Take care of the new devotees so that they may read our literatures carefully and come to the real standard of understanding. Our strength is not material strength, but spiritual strength.
(Letter to Jagadisa---29th. September, 1970).

The secret of preaching work is that one must keep himself fit in spiritual strength by maintaining always the highest standard of purity in Krsna consciousness.
(Letter to Vaikunthanatha---12th. January, 1971).