I think there is a need to understand the difference between the Quranic usage of "muslim" and "momin".
Muslim as we all know is one who surrenders, a very basic level of faith.
Momin is commonly translated into "believer" wich is misleading. For example one can believe in Satan, but he cannot be a "momin" to Satan. A better translation is perhaps "faithful"
A Momin is a rank higher than a muslim as illustrated by the Quranic verse:
قَالَتِ الْأَعْرَابُ آمَنَّا قُل لَّمْ تُؤْمِنُوا وَلَكِن قُولُوا أَسْلَمْنَا وَلَمَّا يَدْخُلِ الْإِيمَانُ فِي قُلُوبِكُمْ
The dwellers of the desert say: We believe. Say: You do not believe but say, We submit; and faith has not yet entered into your hearts;
Note when the A'raab or nomadic arabs claimed to be "momin" they were told to refrain and simply claim the status of "muslim".
