aho danava siddho ’si
yasya te matir idrsi
Translation by His Divine Grace A. C. Bhaktivedanta Swami Srila Prabhupada:
Indra said: O great demon, I see by your discrimination and endurance in devotional service, despite your dangerous position, that you are a perfect devotee of the Supreme Personality of Godhead, the Supersoul and friend of everyone.
Purport by His Divine Grace A. C. Bhaktivedanta Swami Srila Prabhupada:
As stated in Bhagavad-gita (6.22):
yam labdhva caparam labham
manyate nadhikam tatah
yasmin sthito na duhkhena
“Established in Krsna consciousness, one never departs from the truth, and upon gaining this he thinks there is no greater gain. Being situated in such a position, one is never shaken, even in the midst of the greatest difficulty.” An unalloyed devotee is never disturbed by any kind of trying circumstance. Indra was surprised to see that Vrtrasura, undisturbed, was fixed in devotional service to the Lord, for such a mentality is impossible for a demon. However, by the grace of the Supreme Personality of Godhead, anyone can become an exalted devotee (striyo vaisyas tatha sudras te ’pi yanti param gatim). An unalloyed devotee is sure to return home, back to Godhead.