SB 6.17.38
jajñe tvastur daksinagnau
danavim yonim asritah
vrtra ity abhivikhyato
jñana-vijñana-samyutah
Translation by His Divine Grace A. C. Bhaktivedanta Swami Srila Prabhupada:
Being cursed by mother Durga [Bhavani, the wife of Lord Siva], that same Citraketu accepted birth in a demoniac species of life. Although still fully equipped with transcendental knowledge and practical application of that knowledge in life, he appeared as a demon at the fire sacrifice performed by Tvasta, and thus he became famous as Vrtrasura.
Purport by His Divine Grace A. C. Bhaktivedanta Swami Srila Prabhupada:
The word yoni is generally understood to mean jati — family, group or species. Although Vrtrasura appeared in a family of demons, it is clearly said that his knowledge of spiritual life still existed. Jñana-vijñana-samyutah: his spiritual knowledge and the practical application of that knowledge in life were not lost. Therefore it is said that even if a devotee falls down for some reason, he is still not lost.
yatra kva vabhadram abhud amusya kim
ko vartha apto ’bhajatam sva-dharmatah
(Bhag. 1.5.17)
Once one is advanced in devotional service, his spiritual assets are never lost under any circumstances. Whatever spiritual advancement he has achieved continues. This is confirmed in Bhagavad-gita. Even if a bhakti-yogi falls, he takes birth in a rich family or family of brahmanas, in which he again starts devotional activities from the point where he left off. Although Vrtrasura was known as an asura, or demon, he did not lose his consciousness of Krsna or devotional service.