The definition of a pure devotee, as given by Rūpa Gosvāmī in Bhakti-rasāmṛta-sindhu, can be summarized thus: his service is favorable and is always in relation to Kṛṣṇa. In order to keep the purity of such Kṛṣṇa conscious activities, one must be freed from all material desires and philosophical speculation. Any desire except for the service of the Lord is called material desire. And "philosophical speculation" refers to the sort of speculation which ultimately arrives at a conclusion of voidism or impersonalism. This conclusion is useless for a Kṛṣṇa conscious person. Only rarely by philosophical speculation can one reach the conclusion of worshiping Vāsudeva, Kṛṣṇa. This is confirmed in the Bhagavad-gītā itself. The ultimate end of philosophical speculation, then, must be Kṛṣṇa, with the understanding that Kṛṣṇa is everything, the cause of all causes, and that one should therefore surrender unto Him. If this ultimate goal is reached, then philosophical advancement is favorable, but if the conclusion of philosophical speculation is voidism or impersonalism, that is not bhakti.
Karma, or fruitive activities, are sometimes understood to be ritualistic activities. There are many persons who are very much attracted by the ritualistic activities described in the Vedas. But if one becomes attracted simply to ritualistic activities without understanding Kṛṣṇa, his activities are unfavorable to Kṛṣṇa consciousness. Actually, Kṛṣṇa consciousness can be based simply on hearing, chanting, remembering, etc. Described in the Śrīmad-Bhāgavatam are nine different processes, besides which everything done is unfavorable to Kṛṣṇa consciousness. Thus, one should always be guarding against falldowns.