(You) who believed in Our Ayat and were Muslims.
means, their hearts believed and they submitted inwardly and outwardly to the Laws of Allah.
Al-Mu`tamir bin Sulayman narrated that his father said:
"When the Day of Resurrection comes and the people are resurrected, there will be no one left who will not be filled with terror. Then a caller will cry out:
يَا عِبَادِ لاَا خَوْفٌ عَلَيْكُمُ الْيَوْمَ وَلاَا أَنتُمْ تَحْزَنُونَ
My servants! No fear shall be on you this Day, nor shall you grieve.
So all the people will be filled with hope, but this will be followed by the words:
الَّذِينَ امَنُوا بِأيَاتِنَا وَكَانُوا مُسْلِمِينَ
(You) who believed in Our Ayat and were Muslims. Then all of mankind will be filled with despair apart from the believers."