faith and reassurance

إِذْ تَقُولُ لِلْمُؤْمِنِينَ أَلَن يَكْفِيَكُمْ أَن يُمِدَّكُمْ رَبُّكُم بِثَلَـٰثَةِ ءَالَـٰفٍ مِّنَ ٱلْمَلَـٰٓئِكَةِ مُنزَلِينَ

[Remember] when you said to the believers, "Is it not sufficient for you that your Lord should reinforce you with three thousand angels sent down?

قَالَ كَذَٰلِكِ قَالَ رَبُّكِ هُوَ عَلَىَّ هَيِّنٌ ۖ وَلِنَجْعَلَهُۥٓ ءَايَةً لِّلنَّاسِ وَرَحْمَةً مِّنَّا ۚ وَكَانَ أَمْرًا مَّقْضِيًّا

He said, "Thus [it will be]; your Lord says, 'It is easy for Me, and We will make him a sign to the people and a mercy from Us. And it is a matter [already] decreed.'"