Reasoning is there if you simply explain that Krishna is full in 6 opulences: strength, knowledge, wealth, beauty, fame, and renunciation. No one can excel Krishna in opulence. Nobody is equal to or greater than Krishna, therefore He is God.
The word for philosophy is tattva darsana - the Bhagavad-gita recommends that one approach a spiritual master who is tattva darsana, one who has seen the truth. This is philosophy.
Letter to Mr. Prem J. Batra, 2 April, 1976
The Absolute Truth is described in different ways by different people according to angle of vision. But the object is the same. There may be different types of religious systems, but the object is Krsna. Somewhere it is openly expressed, and somewhere it is covered.
Lecture on Bhagavad-gita 4.11, Bombay, March 31, 1974
People generally speak, "God is great." That is very good. At least, one accepts God is great. But how He is great and what is the extent of His greatness, if we understand, then our regard and reverence for Krsna increases. Just like we have got some friend, but if we know the opulence of the friend, how great he is... He may be a very big man, very big business magnate or minister. If we know, then our, "Oh, you have got such a nice friend." Similarly, we should try to understand Krsna.
Lecture on Bhagavad-gita 4.10, Bombay, March 30, 1974
Regarding your manner of behavior with your parents who are not in Krishna Consciousness; I may inform you that you should treat four different classes of men in four different ways. A devotee should love God and God's devotees. A devotee should make friendship with devotees. A devotee should try to enlighten innocent persons, and a devotee should reject opposite elements. As father and Mother they should be offered proper respect according to social custom, but you cannot accept their non-Godly instructions. Best thing is, to avoid misunderstanding, to remain silent without any affirmation or negation of their instructions.
Letter to Nandarani and Dayananda, 29 March, 1968
Religion means a culture of the spirit soul. It may be in different way understood in different countries, but the whole idea is to understand the spirit soul. So dharmena hinah pasubhih samanah. If the human society is not very eager to understand the real position of the soul or consciousness, then he is no better than the animals. That is the version of the Vedic, and actually it is so. Our developed consciousness, our developed life should be utilized in this human form of life to understand "What I am." The whole trouble of the human society is because they have forgotten the constitutional position of his self.
Lecture on Bhagavad-gita 2.46-47, New York, 28 March, 1966
We have surrendered to somebody. Analyze everyone. He has somebody superior where he has surrendered. It may be his family, his wife, or his government, his community, his society, his political party. Anywhere you go, the characteristic is to surrender. That you cannot avoid. That was the talk with Professor Kotovsky in Moscow. I asked him, "Now, you have got your Communist philosophy. We have got our Krsna philosophy. Where is the difference in philosophy? You have surrendered to Lenin, and we have surrendered to Krsna . Where is the difference?" Everyone has to surrender. It doesn't matter where he is surrendering. If the surrendering is correct, then the things are correct. If the surrendering is not correct, then things are not correct. This is the philosophy. So we are surrendering.
Lecture on Bhagavad-gita 4.7, Bombay, March 27, 1974
Krsna is one. He can expand Himself in many forms. That is different thing. But Krsna... Advaitam acyutam anadim ananta-rupam [Bs. 5.33]. Ananta haya. He has multi, innumerable forms, but still, He's one, advaita. Advaitam acyutam, He does not fall. Suppose isvarah sarva-bhutanam hrd-dese 'rjuna tisthati [Bg. 18.61], because Krsna has expanded Himself in many millions of forms to stay within the heart of the living being, that does not mean Krsna has fallen. Avyayatma. He's the same Krsna. There is no difference.
Lecture on Bhagavad-gita 4.6, Bombay, March 26, 1974
You have been a little bit perturbed that the demons are eternally condemned, and I thank you very much that you have been compassionate with the demons. That is the sign of Vaisnavism. The Lord may condemn the demons, but because the Vaisnavas are there, there is ample chance for the demons to become blessed by the devotees of the Lord. The devotees of the Lord are so powerful that each and every one of them has the power of delivering many demons, even condemned by the Lord.
Letter to Revatinandana, 25 March, 1970
So far the cucumber pickles: As far as possible we should not offer to the Deity things which are prepared by nondevotees. We can accept from them raw fruits, grains or similar raw things. So far cooking and preparing, that should be strictly limited to the initiated devotees. And aside from this, vinegar is not good; it is tamasic, in the darkness, nasty food. So I think we shall not accept this pickles.
Letter to Kirtanananda, 24 March, 1969
Actually there is no need to give up studies to be a devotee. In America, a number of our students joined with us before they graduated but still they went on to graduate with honors, namely Bhagavan dasa, Trivikrama Maharaja, and others. So just try to convince them in some way.
Letter to Niranjana, 23 March, 1973