not neccesarily, people have differant amounts of faith or iman as you call it, whilst muslims sin, if they could see for example God watching over them, there iman would be so strong all the time they wouldn't miss a prayer or do anything bad. it reminds me of the situation with one of my muslim friends. recently he said "i am a part time muslim" (in that he doesn't practise all the time) and i asked him well do you believe in it and he said yes, im sure calling yourself a part time muslim is a very bad thing in islam and he knows this so why does he do it? what does he gain from even calling himself that? what desire would he have to call himself that? he obviously doesn't have complete faith in islam.