Why a believer is called a believer

قَالَ أَ تَدْرِي يَا رِفَاعَةُ لِمَ سُمِّيَ الْمُؤْمِنُ مُؤْمِناً قَالَ قُلْتُ لَا أَدْرِي قَالَ لِأَنَّهُ يُؤْمِنُ عَلَى اللَّهِ عَزَّ وَ جَلَّ فَيُجِيزُ [اللَّهُ‌] لَهُ أَمَانَهُ.

‘Do you know, O Rafa’at, why the ‘الْمُؤْمِنُ’ Believer is called ‘Momin’?’ I said, ‘I do not know’. He (asws) said: ‘Because he entrusts himself to Allah (azwj) Mighty and Majestic, so Allah (azwj) Authorises safety for him’.