क्षिप्रं भवति धर्मात्मा शश्वच्छान्तिं निगच्छति।कौन्तेय प्रतिजानीहि न मे भक्तः प्रणश्यति॥
kṣipraṃ bhavati dharmātmā śaśvac-chāntiṃ nigacchatikaunteya pratijānīhi na me bhaktaḥ praṇaśyati
Word by word
The verse says
Quickly he becomes someone whose nature is right, and he comes to lasting peace. Know this for certain, Arjuna: nobody devoted to me is lost.
In plain words
Here is the second half of the promise, and it answers the obvious objection.
Kṣipraṃ bhavati dharmātmā — he quickly becomes one whose nature is right. The conduct changes. It is not being waived.
Then Krishna does something unusual. He tells Arjuna to declare it himself.
Where you've felt this · in family life
Your father once told a shopkeeper, in front of you, that you would pay back what you owed by the end of the month. He did not check with you first. You found the money. What got you there was not really the debt, and everybody involved knew it.
Going deeper
Being counted as good comes with something attached, and it arrives one line later.
The previous verse said the person of bad conduct is to be considered good. This one says he quickly becomes so in fact. The declaration is not a legal fiction that leaves the behaviour untouched; it is the first move in a change that the verse insists will actually happen. Read together, they describe being treated as what you are becoming, which is how people are in fact changed.
The last line is the one people carry with them, and the way it is delivered is deliberate. Krishna does not say nobody is lost. He tells Arjuna to say it — you declare it, make the announcement yourself.
Why hand the promise to somebody else to make? Because a promise you have spoken aloud is harder to argue yourself out of at three in the morning. It is no longer something you were told. It is something you said.