Lecture on Srimad Bhagavatam, Canto 9, Chapter 4, Text 28 by Guru-prasad Swami. Audio only.
Being very pleased by the unalloyed devotion of Maharaja Ambarisha, the Supreme Personality of Godhead gave the King His disc, which is fearful to enemies and which always protects the devotee from enemies and adversities.
A devotee, being always engaged in the service of the Lord, may not be expert in self-defense, but because a devotee fully depends on the lotus feet of the Supreme Personality of Godhead, he is always sure of protection by the Lord. Prahlada Maharaja said:
naivodvije para duratyaya-vaitaranyas
A devotee is always merged in the ocean of the transcendental bliss of rendering service to the Lord. Therefore he is not at all afraid of any adverse situation in the material world. The Lord also promises, kaunteya pratijanihi na me bhaktah pranasyati: “O Arjuna, you may declare to the world that the devotees of the Lord are never vanquished.” (Bg. 9.31) For the protection of the devotees, Krishna’s disc, the Sudarsana cakra, is always ready. This disc is extremely fearful to the nondevotees (pratyanika-bhayavaham). Therefore although Maharaja Ambarisha was fully engaged in devotional service, his kingdom was free of all fear of adversity.