"If you have not yet found a spiritual master but are sincere, Krsna will direct you to a bona fide spiritual master. And if you get a bona fide spiritual master, he will take you to Krsna. Krsna is always sitting in your heart as the caitya-guru, the spiritual master within. It is that caitya-guru who manifests Himself externally as the spiritual master. Therefore the spiritual master is the direct representative of Krsna."
Laws of Nature, Chapter 1
"You have problems, but God is within your heart. Isvarah sarva-bhutanam. God is not far away. If you are sincere, God sends you a spiritual master. Therefore God is also called caitya-guru, the spiritual master within the heart. God helps from within and from without. Everything is thus described in the Bhagavad-gita. This material body is like a machine, but within the heart is the soul, and with the soul is the Supersoul, Krsna, who gives directions. The Lord says, "You wanted to do this; now here is the chance. Go and do it." If you are sincere, you say, "Now, God, I want You." Then He will give you directions. "Yes, now you come and get Me like this." This is His kindness. However, if we want something else, that is all right. We can have it. God is very kind. When I want something, He is in my heart directing me and telling me how to have it. So why should He not give directions on how to have a spiritual master? First of all we must again be eager to revive our God consciousness. Then God will give us a spiritual master."
Science of Self-Realization, Chapter 7
"So Krsna is advising that "I am in everyone's heart." You can take advice from Krsna. Krsna is ready. Krsna's another name is caitya-guru. Caitya-guru means the guru who is situated within your heart. Krsna comes out as instructor guru or initiator guru outside, and he is sitting within the heart as caitya-guru. Krsna is ready to help you, help us, every one of us, in two ways: by the external guru and internal guru. Internal guru, He is Krsna Himself, and external guru, His manifestation, the spiritual master. So we should take advantage of two gurus and make our life successful. This is Krsna consciousness movement."
Srila Prabhupada Lecture on Bhagavad-Gita, 09-26-73, Bombay
"The Lord is the Supersoul seated in everyone's heart, and He acts as the caitya-guru, the spiritual master within. However, He gives direct instructions only to the advanced, pure devotees. In the beginning, when a devotee is serious and sincere, the Lord gives him directions from within to approach a bona fide spiritual master. When one is trained by the spiritual master according to the regulative principles of devotional service and is situated on the platform of spontaneous attachment for the Lord (raga-bhakti), the Lord also gives instructions from within."
Srimad-Bhagavatam 4:28:41 Purport
"The jnanis, yogis and karmis cannot expect this direct cooperation of the Lord. They are not able to satisfy the Lord by transcendental loving service, nor do they believe in such service to the Lord. The bhakti process, as performed under the regulative principles of vaidhi-bhakti, or devotional service following the prescribed rules and regulations, is defined by the revealed scriptures and confirmed by great acaryas. This practice can help the neophyte devotee to rise to the stage of raga-bhakti, in which the Lord responds from within as the caitya-guru, or the spiritual master as Superconsciousness. All transcendentalists other than devotees make no distinction between the individual soul and the Supersoul because they miscalculate the Superconsciousness and the individual consciousness to be one and the same. Such miscalculation by the nondevotees makes them unfit to receive any direction from within, and therefore they are bereft of the direct cooperation of the Lord. After many, many births, when such a nondualist comes to sense that the Lord is worshipable and that the devotee is simultaneously one with and different from the Lord, then only can he surrender unto the Lord, Vasudeva. Pure devotional service begins from that point. The process of understanding the Absolute Truth adopted by the misguided nondualist is very difficult, whereas the devotee's way of understanding the Absolute Truth comes directly from the Lord, who is pleased by devotional service."
Srimad-Bhagavatam 3:5:4 Purport
"One who is sincere and pure gets an opportunity to consult with the Supreme Personality of Godhead in His Paramatma feature sitting within everyone's heart. The Paramatma is always the caitya-guru, the spiritual master within, and He comes before one externally as the instructor and initiator spiritual master. The Lord can reside within the heart, and He can also come out before a person and give him instructions. Thus the spiritual master is not different from the Supersoul sitting within the heart. An uncontaminated soul or living entity can get a chance to meet the Paramatma face to face. Just as one gets a chance to consult with the Paramatma within his heart, one also gets a chance to see Him actually situated before him. Then one can take instructions from the Supersoul directly. This is the duty of the pure devotee: to see the bona fide spiritual master and consult with the Supersoul within the heart."
Srimad-Bhagavatam 4:28:52 Purport
"Krsna appears in two ways. He appears as antaryami, the Supersoul within himself, within oneself, and He appears as the spiritual master, externally. Caitya-guru and diksa-guru. Caitya-guru... As caitya-guru, as the supreme spiritual master, He is in everyone's heart. And His representative... In order to help externally the sincere person, He sends the spiritual master. Therefore the spiritual master from outside and the caitya-guru from inside, they are helping. The Supreme Personality is helping a devotee, a real, serious devotee two ways: from within, from without. The Caitanya-caritamrta therefore says that krsnera prakasa, manifestation of Krsna, a spiritual master. And Visvanatha Cakravarti Thakura says, yasya prasadad bhagavat-prasado yasya aprasadad na gatih kuto 'pi: "If I can satisy my spiritual master, then I understand that Krsna is satisfied. And if I cannot satisfy my spiritual master, then I have no other way to approach Krsna." These songs we sing every day. So my Guru Maharaja used to say that "Instead of seeing Krsna, you serve so faithfully that Krsna will see you." That should be the process. Krsna is omnipresent. He can see you, He can understand you, what are doing. So we have to execute devotional service in such a faithful way that Krsna will see us. Then you will be successful. "Don't try to see Krsna. Try to become seen by Krsna." That was my Guru Maharaja's instruction."
Srila Prabhupada Lecture on Srimad-Bhagavatam, 02-10-71, Gorakhpur
Bhaktivedanta Book Trust. HDG A.C. Bhaktivedanta Swami Srila Prabhupada.