When they become fully conversant with the science of Kṛṣṇa consciousness, such saintly souls can act as spiritual masters for all humanity, regardless of any external designation. Lord Caitanya confirms this in the Caitanya-caritāmṛta (CC Madhya 8.128):
kibā vipra, kibā nyāsī, śūdra kene naya
yei kṛṣṇa-tattva-vettā, sei 'guru' haya
Whether one is a brāhmaṇa, a sannyāsī, or a śūdra—regardless of what he is—he can become a spiritual master if he knows the science of Kṛṣṇa.
Therefore we can conclude that neither the karmīs nor the jñānīs can fathom the depths of the science of devotional service to Kṛṣṇa. Especially the foolish karmīs are disqualified, for they generally consider Lord Kṛṣṇa an ordinary mortal, and this disregard for the Lord leads them to misconstrue the meaning of His words in the Bhagavad-gītā.