Caitya-guru means who gives conscience and knowledge from within.
Sunday Feast Lecture -- Los Angeles, May 21, 1972: In the Vedānta-sūtra or Bhāgavata it is said that Brahmā... He's the first creature. There was no other any other living entity when he was created first. So if I say that he also got knowledge from others, then the argument may be, "Who is the next person to give him knowledge?" So therefore Bhāgavata says, "No. He received knowledge from Kṛṣṇa." How? "From the heart." Tene brahma hṛdā. Hṛdā. Because God, Kṛṣṇa, is sitting in everyone's heart—your heart, my heart, everyone. And He can give you instruction. His name is therefore Caitya-guru.
Caitya-guru means who gives conscience and knowledge from within. In the Bhagavad-gītā Kṛṣṇa says, sarvasya cāhaṁ hṛdi sanniviṣṭo: [Bg. 15.15] "In everyone's heart I am sitting." Hṛdi, "within the heart"; sanniviṣṭo, "I am sitting there." Sarvasya. Not only you and me, even animals insects, birds, beasts, Brahma, everyone. Sarvasya. All living entities. So sarvasya cāhaṁ hṛdi sanniviṣṭo mattaḥ: "from Me"; smṛtir jñānam apohanaṁ ca, "remembrance, knowledge and forgetfulness." Forgetfulness also. If you want to forget God, God will give you intelligence that you can forget God forever. He's so kind. Whatever you want, He will give you intelligence, "Do like this."