إن الذين آمنوا
Surely, those who believe,
referring to Muslims,
والذين هادوا
those who are the Jews,
who were entrusted with the Tawrah,
والصابوون
and the Sabians...
a sect from the Christians and Magians who did not follow any particular religion, as Mujahid stated.
والنصارى
and the Christians,
As for the Christians, they are known and were entrusted with the Injil.
من آمن بالله واليوم اﻻخر وعمل صالحا فﻼ خوف عليهم وﻻ هم يحزنون
whosoever believed in Allah and the Last Day, and worked righteousness, on them shall be no fear, nor shall they grieve.
The meaning here is that if each of these groups believed in Allah and the Hereafter, which is the Day of Judgment and Reckoning, and performed good actions, which to be so, must conform to Muhammad's Law, after Muhammad was sent to all mankind and the Jinns. If any of these groups held these beliefs, then they shall have no fear of what will come or sadness regarding what they lost, nor will grief ever affect them.
We discussed a similar Ayah before in Surah Al-Baqarah (2:62)