Tafsir of Al-Ahzab 33:15

ولقد كانوا عاهدوا الله من قبل ﻻ يولون اﻻدبار

And if the enemy had entered from all sides, and they had been exhorted to Al-Fitnah, they would surely have committed it and would have hesitated thereupon but little. And indeed they had already made a covenant with Allah not to turn their backs, Allah tells us about those who said:

يقولون إن بيوتنا عورة وما هي بعورة إن يريدون إﻻ فرارا

("Truly, our homes lie open.'' And they lay not open. They but wished to flee). If the enemy had entered upon them from every side of Al-Madinah and from every direction, then demanded that they become disbelievers, they would have promptly done so. They would not have adhered to their faith or remained steadfast in it if they were faced with the slightest fear. This is how it was interpreted by Qatadah, Abdur-Rahman bin Zayd and Ibn Jarir. This is a condemnation of them in the clearest terms. Then Allah reminds them of the promise they had made to Him before this fear struck them, that they would not turn their backs or flee from the battlefield.

وكان عهد الله مسووﻻ

and a covenant with Allah must be answered for. means, Allah will inevitably ask them about that covenant