अथ चैत्त्वमिमं धर्म्यं संग्रामं न करिष्यसि। ततः स्वधर्मं कीर्तिं च हित्वा पापमवाप्स्यसि ॥

atha chet tvam imaṁ dharmyaṁ saṅgrāmaṁ na kariṣhyasi tataḥ sva-dharmaṁ kīrtiṁ cha hitvā pāpam avāpsyasi

But if thou wilt not carry on this righteous warfare, then casting away thine own duty and thine honour, thou wilt incur sin.

In plain words: Krishna warns that if Arjuna refuses to fight this righteous battle, he will give up his duty and his honor, and that would be a sin.