The difference between material world and spiritual world means in the material world we have forgotten Krsna. Material means... When we forget God, that is material. And when we know God, existence of God, then that is spiritual. This is the difference between material and spiritual.
Lecture on Bhagavad-gita 9.5, Melbourne, April 24, 1976
Materialism does not mean that one has to possess so many things, the actual fact is one may be a perfect transcendentalist of spiritual man by possessing the whole world, and one may be a gross materialist without possessing a farthing. So this distinction can be made on the basis of consciousness. When one is Krishna Conscious possessing everything in the world, he is perfect spiritualist, and one may have renounced everything in the world, but lacking in Krishna Consciousness he is a gross materialist.
Letter to Sri Birlaji, 23 April, 1970
We are the best peaceful group in the world and we are trying to bestow upon the people the supermost knowledge for the humanity. Everyone should cooperate with us, but unfortunately this is the age of Kali and people are not very much eager to accept genuine things, so don't be disappointed. Do as you are doing now and Krsna will help you without fail. To face difficulty in the course of preaching work is a kind of austerity. These difficulties do not go in vain.
Letter to Upendra, 22 April, 1970
I am very glad to learn that you are doing spiritual activities very nicely and you are always alert to see whether you are committing some offense. This is a very nice attitude. In rendering service to Lord Krsna and His representative the Spiritual Master, we should always maintain this fearful attitude which means careful attention. This attitude will advance you progressively in Krishna Consciousness.
Letter to Gopala Krishna, 21 April, 1970
As long as a man is in the full vigor of life, he forgets the naked truth of death, which he has to meet.
“Back to Godhead”, April 20, 1960
Devotional service must continue under all circumstances. This material body is actually a bad bargain because it is prone to suffer, but we must make the best use of this bad bargain. That means to always be engaged in the service of Krishna without fail.
Letter to Viruha dasa, 19 April, 1975
The atmosphere contains all the elements needed to sustain life: earth, water, fire, air and ether. In anything material, all these elements are present. For example, in my body there is water, although you cannot see it. Similarly, you don't see fire in my body, yet my body is warm. Where does this warmth come from? You don't see any fire. Do you see any fire burning in my body? Then where does the warmth come from? What is the answer?
Morning walk, Cheviot Hills Park, Los Angeles, April 18, 1973
Actually Hindus and non-Hindus everyone is at the present moment out of touch of the real science of spiritual knowledge. Everyone is going under some religious badge only, so it is very difficult to deal with them unless they are very much serious to understand the science of God. Everyone of the Hindu community in the Western world has got some very good feeling for me because superficially they are seeing that I am spreading Hindu religion, but factually this Krishna Consciousness movement is neither Hindu religion nor any other religion. It is the function of the soul.
Letter to Jayapataka, 17 April, 1970
The chief aim for learning Sanskrit would be how to pronounce the Sanskrit verses especially in our published books just like you have already chanted the Govinda verses in the record. Similarly all the verses you have to chant combinedly and melodiously along with musical instruments, and it will be a great charm to the people of the world.
Letter to Gurudasa, 16 April, 1970
I have passed my M.A. examination, and in the university, in the convocation meeting, I get the degree and people applauses me; but if one is situated in consciousness, he will understand, "What is this degree? This degree is due to my body. As soon as this body finishes, all these degree will finish. Because, if it is a fact that vasamsi jirnani yatha vihaya [Bg. 2.22], if I have to take another body after leaving this body, then everything in bodily connection will finish as soon as I give up this body and take up another body. That is my position.
Lecture on Bhagavad-gita 2.55-58, New York, April 15, 1966