So when they went back to their father, having purified their preparedness and been trained by the configurations of virtues they required of him that he send forth with them the practical rational faculty to supply them permanently with virtuous character traits by means of meanings. In other words, they sought to be supplied from his effusion; that we may obtain the measure, that we may profit from him. For surely we shall not use him to actualise our goals and bring about his perdition, as we did, during the time of ignorance, with his brother [Joseph]. Rather, we will guard him well by obligating ourselves to him and being mindful of him on the path to perfection.
The taking from them of a covenant and a pledge in sending him forth with them expresses the precedence given to the sound conviction of faith over deeds and obligating them to that covenant first. Otherwise their state as they embarked on the task would not have been proper and would not have had a wholesome effect.