Krishna follows through on the promise of 9.30. That person of terrible past conduct who turns to bhakti? They do not stay where they were. Quickly — kshipram — they become a dharmatma, a person of righteousness. And they attain peace that does not fade.
Then comes the guarantee. Krishna tells Arjuna: 'Pratijanihi' — make this declaration on my behalf — my devotee never perishes. This is not advice. It is a personal pledge from Krishna. He does not even make it himself in the first person — he asks Arjuna to announce it, as if placing his own reputation in his devotee's hands.
There is a reason Krishna asks Arjuna to declare it rather than saying it directly. If Krishna says it, one might think he is boasting. But when Arjuna declares it, it becomes a witnessed vow — unbreakable.