The Letter To The Church In Philadelphia (Revelation 3:7-13).
‘And to the angel of the church in Philadelphia write, These things says he who is holy, he who is true, he who has the key of David, he who opens and none shall shut, who shuts and none opens.'
‘He who is holy.' The ‘Holy One' is a title of God (e.g. Isaiah 57:15; Hosea 11:9; Habakkuk 3:3), and in Isaiah He is regularly called ‘the Holy One of Israel' (Isaiah 12:6 and often). It refers to His unique distinctiveness, His ‘otherness', distinctive in essence and in total purity. It is a title bestowed on Christ (Psalms 16:10; Acts 3:14; 1 John 2:20).
‘He who is true.' In 1 John 5:20 God is called ‘Him who is true', and here the description is applied to Christ. In Revelation 6:10 Christ is called ‘the holy and true' as here. The sense of ‘true' is that He is real and reliable and the source of truth.
‘He who has the key of David, he who opens and none shall shut, who shuts and none shall open.' This picture is taken from Isaiah 22:20-25. There in the days of Hezekiah the key of David is to be given to Eliakim, who will replace the false chief steward of the royal palace, the treasurer over the king's treasury. ‘And he will be a father to the inhabitants of Jerusalem and to the house of Judah, and the key of the house of David will I lay on his shoulder, and he shall open and none shall shut, and he shall shut and none shall open'. Eliakim was a faithful steward who managed His stewardship well and was seen as a father to the people. So Jesus too will be faithful and true in watching over His people and providing for His own. None can prevent it. But He will be faithful, not as the steward, but as the son over His own house (Hebrews 3:6). The idea of the key of David may well suggest that He controls access to the New Jerusalem (See below, Revelation 3:12, and Revelation 21:2).
However, it will be noted that this is the only introduction which apparently does not refer back to chapter 1 for its source. If it was so this must be considered somewhat surprising. Thus we must consider the suggestion that the mention of a key ties in with the Son of Man as having the keys of the after-world and of death (Revelation 1:18). This would stress that He has power over the grave and can release or retain whom He will. If He opens, none can shut. If He shuts none can open.
So the church of Philadelphia are called on to recognise that as the greater David He controls the afterlife and death, releasing whom He will, and that He can provide, or refuse, access to the New Jerusalem.