The guru is one who knows Kṛṣṇa-tattva, whatever his varṇa or āśrama
kibā vipra, kibā nyāsī, śūdra kene naya
yei kṛṣṇa-tattva-vettā, sei 'guru' haya"whether a brāhmaṇa, a sannyāsī or a śūdra — one who knows the science of Kṛṣṇa is a guru" — CC Madhya 8.128