Bhaktya kevalaya means
SB Canto 7
Nārāyaṇa, the Supreme Personality of Godhead, who is full of all opulences, is predominant within the core of your heart because of your being a pure devotee. He always drives away all the darkness of ignorance, as the sun drives away the darkness of the universe.
The words bhaktyā kevalayā indicate that simply by executing devotional service one can become full of all knowledge. Kṛṣṇa is the master of all knowledge (aiśvaryasya samagrasya vīryasya yaśasaḥ śriyaḥ (Viṣṇu Purāṇa 6.5.47)). The Lord is situated in everyone's heart (īśvaraḥ sarva-bhūtānāṁ hṛd-deśe 'rjuna tiṣṭhati (BG 18.61)), and when the Lord is pleased with a devotee, the Lord instructs him. Only to the devotees, however, does the Lord give instructions by which to advance further and further in devotional service. To others, the nondevotees, the Lord gives instructions according to the manner of their surrender. The pure devotee is described by the words bhaktyā kevalayā. Śrīla Viśvanātha Cakravartī Ṭhākura explains that bhaktyā kevalayā means jñāna-karmādy-amiśrayā, "unmixed with fruitive activities or speculative knowledge." Simply surrendering at the lotus feet is the cause of all a devotee's enlightenment and awareness.