"Guru and Krishna are two parallel lines on which the spiritual express runs very smoothly. In the Chaitanya Charitamrita it is said 'Guru Krishna prasade payae bhakti lata vija.' By the Grace of Guru one gets Krishna and by the Grace of Krishna one gets a bonafide Guru.
Therefore Krishna consciousness means staunch faith both in Guru and Krishna. One minus the other is no good for the devotee. So your faith in the principles of devotion to Guru will certainly help you more and more Krishna. Do not ever try to approach Krishna directly. Anyone who talks of Krishna without service to Guru will not be successful. So your faith in Guru and Krishna simultaneously will help you to become crowned with success in the progressive march in Krishna consciousness. Do not be worried, be situated in the present aptitude and everything will be clear."
(Srila Prabhupada Letter, September 27, 1967)
"There are two gurus—one internal and the other external. The internal Guru is Krishna Himself seated in everyone's heart, and the external Guru is the Spiritual Master. So a sincere devotee is helped both externally and internally. To the sincere devotee the internal Guru Krishna dictates, but the thing has to be confirmed by the external Guru then it is all right."
(Srila Prabhupada Letter, January 14, 1968)
"The Spiritual Master and Krishna are two parallel lines. You have to make your progress on these two parallel lines, you cannot avoid one in preference of the other. The train, on two tracks, moves forward. The Spiritual Master and Krishna are like these two tracks, they must be served simultaneously. Krishna helps one to find bona fide Spiritual Master, and bonafide Spiritual Master helps one to understand Krishna. If one does not get bonafide Spiritual Master, then how he can ever understand Krishna? You cannot serve Krishna without Spiritual Master, or serve just Spiritual Master without serving Krishna. They must be served simultaneously."
(Srila Prabhupada Letter, February 12, 1968)
"Krishna is so kind that He wants to help us as Spiritual Master in two ways. He helps us from within as Caitya Guru and He expands Himself externally as Siksa Guru (as instructor) and diksa Guru (initiator). So the principle is that whatever you are instructed by the Caitya Guru internally may be confirmed by the instructor or initiator externally. Then your progress will be complete."
(Srila Prabhupada Letter, May 21, 1969)
"Krishna is the original Spiritual Master known as Chaitya Guru and He manifests Himself as the Instructor Spiritual Master. So this principle of following Krishna and Guru simultaneously is the secret of success, and if you follow this policy in the chain of disciplic succession than there is no doubt about your final achievement."
(Srila Prabhupada Letter, January 21, 1970)
"Perhaps you remember the verse in the tenth chapter of Bhagavad-gita that the Lord Himself supplies the light and intelligence from within the heart of a devotee who is constantly engaged in His transcendental loving service. So, the secret of success is that one should be eager to serve the Spiritual Master and Krsna simultaneously with the same regard and things will become automatically revealed in the heart of the devotee."
(Srila Prabhupada Letter, January 26, 1970)
"Kṛṣṇa can become the director and spiritual master of anyone who is serious about getting the mercy of Kṛṣṇa. The Lord sends the spiritual master to train a devotee, and when the devotee is advanced, the Lord acts as the spiritual master within his heart.
dadāmi buddhi-yogaṁ taṁ
yena mām upayānti te
'To those who are constantly devoted and worship Me with love, I give the understanding by which they can come to Me.' Kṛṣṇa does not become the direct spiritual master unless one is fully trained by His representative spiritual master. Therefore, as we have already discussed, the Lord’s representative spiritual master should not be considered an ordinary human being. The representative spiritual master never gives any false knowledge to his disciple, but only perfect knowledge. Thus he is the representative of Kṛṣṇa. Kṛṣṇa helps as the guru, or spiritual master, from within and from without. From without He helps the devotee as His representative, and from within He talks personally with the pure devotee and gives him instructions by which he may return home, back to Godhead."
(Śrīmad-Bhāgavatam 7.15.76, Purport)
"It is not possible for a conditioned soul to directly meet Kṛṣṇa, the Supreme Personality of Godhead, but if one becomes a sincere devotee and seriously engages in devotional service, Lord Kṛṣṇa sends an instructing spiritual master to show him favor and invoke his dormant propensity for serving the Supreme. The preceptor appears before the external senses of the fortunate conditioned soul, and at the same time the devotee is guided from within by the caittya-guru, Kṛṣṇa, who is seated as the spiritual master within the heart of the living entity."
(Caitanya-caritāmṛta Ādi 1.58, Purport)
"Kṛṣṇa is so kind. Guru-kṛṣṇa-kṛpā. Kṛṣṇa is helping from within, but sometimes we are so dull, naturally, that we cannot understand. Therefore He sends His representative to instruct externally. So He is helping internally and externally. There is no difference between the internal and external instructor. We should take advantage of this instruction. That is called vyavasāyātmikā buddhiḥ, fixed-up resolution. Fixed-up resolution. If we become fixed up in this resolution, that 'Whatever we have heard from my guru, the representative of Kṛṣṇa, I must execute. I do not care for my personal convenience or inconvenience. This is my life and soul,' then your life is perfect. Then your life is... If I make some amendment, addition, alteration, in the name of Kṛṣṇa, guru, then it is spoiled. No. We should receive the instruction as it is."
(Srila Prabhupada Lecture, Mayapur, October 28, 1974)