If you do not take your initiation mantra from the sampradāya, then it is useless. Now, there are so many apasampradāya. They do not come in disciplic succession, but becomes guru, teacher. Therefore everything is topsy-turvied. Nobody has got fixed idea what is God. Everyone has created his own philosophy. The whole thing is now confused.
Therefore Kṛṣṇa specifically mentions, Brahma-sūtra. Brahma-sūtra-padaiś caiva hetumadbhir viniścitaiḥ (BG 13.5). The sampradāya must have understanding of the Brahma-sūtra, Vedānta-sūtra. So all the sampradāyas, they have got their commentary on the Vedānta-sūtra and... Even Śaṅkarācārya. But his commentary is not accepted by the Vaiṣṇava ācāryas because he has tried to derive some meaning, interpretation. But there is no question of interpretation. When the things are clear, in the Brahma-sūtra, all the sūtras are very clear. So you don't require any interpretation. You can expand, explain very elaborately. That is another thing. But you cannot go beyond the sutra.
Śrī Caitanya Mahāprabhu therefore says, māyāvādī bhāṣya sunile haya sarva nāśa (CC Madhya 6.169). Māyāvādī bhāṣya means Śaṅkara, Śārīraka-bhāṣya of the Brahma-sūtra. If you hear the Śārīraka-bhāṣya, then you'll be doomed, you will be Godless. Therefore it has been forbidden by Caitanya Mahāprabhu. All the Vaiṣṇava sampradāyas, Rāmanujya Sampradāya, Madhvācārya Sampradāya, they all, I mean to say, disagree with the commentary of Śaṅkarācārya, Śārīraka-bhāṣya, Brahma-sūtra.
So far we are concerned, Madhva-Gauḍīya Sampradāya, our ācāryas, they took it, Śrīmad-Bhāgavatam, as the right commentary on Brahma-sūtra. Bhāṣyaṁ brahma-sūtrānāṁ vedārtha-paribṛṁhitam **. This Śrīmad-Bhāgavatam is the real bhāṣya of Brahma-sūtra. So the Gauḍīya Sampradāya did not make any commentary on the Brahma-sūtra because they took it, Caitanya Mahāprabhu took it as, Śrīmad-Bhāgavatam, as a natural commentary, because Śrīmad-Bhāgavatam is also made by Vyāsadeva and Vyāsadeva is the original author of Brahma-sūtra. So author made his own commentary; so there was no need of another commentary. This is the Gauḍīya-siddhānta, Gauḍīya-vaiṣṇava-siddhānta.