Al-Baqarah: 62
"Indeed, those who believed..."
Those who believed with their tongues without the agreement of their hearts; they are the hypocrites.
"And those who were Jews..."
Those who became Jews. It is said: hāda, yahūdu, tahawwada (he became a Jew) if he enters into Judaism. He is a hā’id, and the plural is hūd.
"And the Christians..."
This is the plural of naṣrān. It is said: a man is naṣrān and a woman is naṣrānah. It is said: "A naṣrānah who has not turned to the true faith (ḥanīf)." The yā’ in naṣrānī is for intensification, like the one in aḥmarī. They were named so because they aided (naṣarū) the Messiah.
"And the Sabians..."
This is derived from ṣaba’a, meaning to exit from a religion. They are a people who deviated from the religion of Judaism and Christianity and worshipped the angels.
"...whoever believes..."
From among these disbelievers, [whoever believes] with a pure faith and enters into the religion of Islam with a genuine entry.
"...and does righteousness, then for them is their reward..."
That which they deserve by virtue of their faith and their deeds.
Grammatical Note:
If you ask: "What is the grammatical position of man āmana (whoever believes)?"
I say: It is in the nominative case (rafʿ) if you make it the subject (mubtada’), with its predicate being falahum ajruhum (then for them is their reward). It is in the accusative case (naṣb) if you make it a substitute (badal) for the noun of inna (the alladhīna that precedes it). In the first case, the predicate of inna is the entire sentence, and in the second case, it is falahum ajruhum, with the fā’ included due to the implication of a conditional meaning.