The name Philadelphia means brotherly love: it was a favoured church.
Philadelphia was known in old times as the door to the east: it was built on one of the great Roman roads which thrust its way like an arrow into the heart of the interior, leading to important regions further east such as Mysia, Lydia and Phrygia. The surrounding country was covered with vineyards and although they had temples dedicated to various gods, the main one was dedicated to Dyonisius, the god of wine.
As in the other letters to the churches, the Lord Jesus introduces himself in a way that has a direct bearing on the character of the church. In this one he is the holy and true holder of the key of David:
The Holy: he is God, for this is one of the names given to God in the Bible (e.g. Isaiah 40:25, I Peter 1:15).
The True: he is the Son of God (I John 5:20), and in him is all truth (John 14:6), the perfection of righteousness and the fulfilment of prophecy.
The holder of the key of David: in 1:18 he held the keys of the kingdom of heaven to give his disciples, and here he tells of the fulfilment of a prophecy found in Isaiah 22:20-25; as Eliakim was given full authority over Jerusalem, so Christ received from God all power in heaven and on earth (Matthew 28:18), and as a Son over his own house, the church (Hebrews 3:6).
Knowing their works, with his authority he places before this church an "open door". The door of salvation cannot be meant here, for this is always open, and is represented in the Bible as the Lord himself (John 10:1-9). The open door mentioned here must be the door of opportunity, of which Christ has the key (Isaiah 45:1-8, Acts 12:10, 14:27). Opportunity involves service of various kinds, such as witness, preaching the Gospel and teaching, and it is only the Lord Jesus who opens and closes it. The reason the door was opened to this church was because:
it had no strength: maybe it was small and without influence in the community; it knew it was weak, yet the Lord's strength is made perfect in weakness (II Corinthians 12:9), and through faith men in the past, out of weakness, were made strong (Hebrews 11:34).
it kept God's word: it learned and obeyed it faithfully.
it did not deny the name of Christ: it was not ashamed, nor afraid, of being known as a follower of the Lord Jesus (Acts 4:12).
Two promises are made:
conversion of Jews: as in Smyrna, there was a synagogue which rejected the Gospel (Hosea 1:8-9, 2:23). They would, however, be converted because of the witness of the church.
they would be kept from the great tribulation: this is a trial which shall come upon the whole world, as judgment. We are told about it from chapter 6, and this promise confirms that the faithful church will not be present.
The church is exhorted to hold fast what you have, i.e., not to relinquish the Word of God, and the testimony of Christ (1:2), because he comes quickly (2:5); the crown mentioned here must be the crown of victory (II John 8,9).
The church in Philadelphia is a type of the majority of the churches from about 1750 AD to 1940 AD. Churches which were small and of little influence (compared with the powerful and dead religious institutions which came out of the Roman Catholic System in the previous period), but characterised by a desire to learn and faithfully obey God's Word, and to practice charity and brotherly fellowship, found that an open door was placed before them: there was virtually no place where a missionary could not go; it was a period of great missionary movement, when the Gospel was taken to China, Japan, Korea, India, Africa, Central and South America, and of great "revivals" in Europe and North America. The word evangelical was applied to them, because of their zeal in spreading the Gospel of Christ. During this time Jewish missions first began in Germany, took root in England, and spread to flourish in the United States; by 1900 it is estimated that some 250,000 Jews came to Christ. Since the second World War missionary zeal has waned, and missionaries are no longer welcomed in an increasing number of countries.
In this letter, the promises made to he who overcomes are that he will be an integral part forever of God's church (II Corinthians 6:16), the property of God, a citizen of the New Jerusalem, and be identified with Christ at his second coming (19:12).
7 "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 no one shuts, and shuts and no one opens":
8 "I know your works. See, I have set before you an open door, and no one can shut it; for you have a little strength, have kept My word, and have not denied My name.
9 "Indeed I will make those of the synagogue of Satan, who say they are Jews and are not, but lie - indeed I will make them come and worship before your feet, and to know that I have loved you.
10 "Because you have kept My command to persevere, I also will keep you from the hour of trial which shall come upon the whole world, to test those who dwell on the earth.
11 "Behold, I am coming quickly! Hold fast what you have, that no one may take your crown.
12 "He who overcomes, I will make him a pillar in the temple of My God, and he shall go out no more. And I will write on him the name of My God and the name of the city of My God, the New Jerusalem, which comes down out of heaven from My God. And I will write on him My new name.
13 "He who has an ear, let him hear what the Spirit says to the churches." '
Rev 3:7-13 (NKJV)