The Brahmā is the original creature. Therefore he has got contact with the original source. He has got his original source how he receives knowledge. Because we have got understanding that we get knowledge from another person when we are face to face. But Brahmā was alone. How he got the knowledge?
That is explained in the Bhāgavatam: tene brahma hṛdā. Hṛdā means through the heart. Because the Supreme Person, Paramātmā, is also within the heart. So although Brahmā was alone, he was getting the knowledge, dictation, from the Supreme. Tene brahma. Brahma means knowledge. Brahma means Veda. The Vedic knowledge was given to him first. Tene brahma hṛdā.
The Vedic knowledge is given to everyone, because Kṛṣṇa is in everyone's heart. Sarvasya cāhaṁ hṛdi sanniviṣṭaḥ (BG 15.15). He's sitting in everyone's heart. But one must be qualified to receive the knowledge. Kṛṣṇa is helping us from within. He is giving us knowledge from within. Without also. So without, He's spiritual master. And within He's Supersoul, caitya-guru. In this way, we are getting knowledge.
So Brahmā is getting knowledge from Kṛṣṇa, and Brahmā is distributing Vedic knowledge. Therefore he's authority. We, we belong to the Brahma-sampradāya, our this Caitanya Mahāprabhu's sampradāya. There are four sampradāyas: one from Brahmā; one from Śrī, Lakṣmī; one from Śambhu, Lord Śiva; and one from Kumāra. Kumāraḥ kapilo manuḥ. So there are four Vaiṣṇava sampradāyas. So we have to approach the authoritative representative of Brahmā, Kṛṣṇa. Then we can get the real knowledge.