From W.E. Vine's Expository Dictionary of New Testament Words we read that
ναὸν naos ...the sanctuary in the Temple; into which only the priests
could lawfully enter - page 1138
ἱεροῦ hieron ...distinct from the 'naos', the inner sanctuary - page
Josephus used hieron to describe the whole Temple area, including the outer courts. Abingdon's Interpreter's Dictionary, under the heading 'Temple' explains:
"Two words are used in the Greek: ναὸν naos, meaning the sacred
building alone, and ἱεροῦ hieron, the whole sacred area, including
various auxiliary courts, side chambers, and porticos. Both words are
translated 'temple' (in the King James Version) without distinction,
but the reader needs to keep the difference in mind, especially as
only priests could enter the 'naos'." (Vol. II p 551)
A careful check needs to be kept on different translations of the Bible as some are consistent, while others are not. Naos is consistently rendered as 'sanctuary' in Robert Young's Literal Translation of the Holy Bible. But it is rendered four different ways in the New World Translation (as temple, temple sanctuary, sanctuary and shrine). The NWT renders hieron as temple 71 times, and naos as temple 24 times. There's always a possibility of confusion. The two words need to be understood as to their particular applications to the Temple, both earthly ones, and the Temple (Sanctuary) of God in heaven.
In Herod's Temple in Jerusalem, the naos consisted of the Holy of Holies, the Holy Place, the Porch, the Altar of burnt offering, and the Court of Priests. The hieron incorporated the Court of Israel, the Sanctuary Gates, the Nicanor Gate (Women's Court) and the Gate Beautiful and the Women's Gallery.
Hieron occurs 71 times in the New Testament. For example, in John 10:23 the Colonnade of Solomon was in the hieron, as was the Temple gate called Beautiful (Acts 3:2), and indeed so were the money-changers, driven out of the heiron by Jesus (Matthew 21:12). But Jesus was never said to have been in the naos on earth. He is always in the hieron. The disciples are always found preaching in the hieron, never in the naos.
When John had his heavenly vision of the 'great crowd', he saw them rendering sacred service to God in his sanctuary, ναὸν naos. In Revelation chapter 7, the great crowd are inside the holy dwelling of God in heaven, in the very centre of God's heavenly sanctuary. John did not see them in some external courtyard. They were not outside of the presence of God on his throne in heaven.
Revelation uniquely has the expression 'before the throne' (Greek, enopion tou thronou), nine times. Enopion is formed from en which means 'in' and ops which means 'the eye'. So enopion means that which is before or opposite a person, towards which he turns his eyes, that which is in one's sight. Therefore, every time the Revelation speaks of objects, angels, or the redeemed being 'before the throne' of God, which is in heaven, they are likewise in heaven, in sight of God.
Naos occurs 16 times in Revelation, but hieron is never mentioned, which is how we know that the great crowd of Revelation 7:9-17 actually stands in heaven, just as in Revelation 14:3 the 144,000 sing a new song before the throne of God, which is in heaven. This is by way of just one example as to why it is important to know the subtle difference of meaning between ἱεροῦ hieron, and ναὸν naos.
Don't close the Door! pp 76-97, Reachout Trust