See commentary on Revelation 2:2.
Θύραν ἐνεῳγμένην [Thyran eneōgmenēn] : a door while having been opened (perfect tense) - the door now stands open after having been opened by Jesus. Although Jesus is knocking at a shut door in Laodicea (Rev. Rev. 3:20+), at Philadelphia He Himself holds the door open such that no one can shut it . This door in Philadelphia could represent the door of evangelism and illumination without which human promulgation of the gospel falls on unreceptive ears (Luke Luke 24:45; Acts Acts 14:27; Acts 16:14; 1Cor. 1Cor. 16:9; 2Cor. 2Cor. 2:12; Col. Col. 4:3). See commentary on Revelation 3:20. In view of the mention of the key of David (Rev. Rev. 3:7+) and synagogue of Satan (Rev. Rev. 3:9+), it seems more likely the door provides entrance into the Messianic Kingdom where Christ will rule from the throne of David (see commentary on Revelation 3:21).1
It speaks of a sure entrance into the Messianic Kingdom, promised to this church as a reward for their faithfulness. No one, not even those of the synagogue of Satan, can shut them out. Jewish opponents would seek to deny Gentiles, such as Christians in this city, entrance into the Messianic Kingdom.2In view of Jesus subsequent promise to the overcomer, I also will keep you from the hour of trial which shall come upon the whole world (Rev. Rev. 3:10+), perhaps this door that is held open and which no man can shut is a sure pathway to heaven for the faithful at the time of the Rapture: After these things I looked, and behold, a door standing open in heaven. And the first voice which I heard was like a trumpet speaking with me, saying, Come up here, and I will show you things which must take place after this. [emphasis added] (Rev. Rev. 4:1+). See Rapture.
a little strength
Even though they have but little strength, they will be able to avail themselves of the door since Christ ensures it remains open. Some believe this should be translated without the indefinite article, little strength as an indication of weakness rather than power.3 The ministry of the church at Philadelphia would be all the more effective because in their strength their accomplishments would be undeniably of God (2Cor. 2Cor. 12:9; Php. Php. 4:13). Others believe that the commendations given to this church are an indication of their spiritual vitality so that this phrase must refer to the churchs limited influence because of its numerical smallness.4
kept My word
The church at Philadelphia had faithfully kept His word. They had preserved its meaning and applied it to their own lives. They were blessed with those who keep the words of this prophecy (Rev. Rev. 1:3+; Rev. 22:7+). It is by keeping his wordand the commandments thereinthat the Philadelphian church demonstrated the depth of their love for Him (John John 14:21-24). In our own day, there are many pressures attempting to dissuade believers from keeping His word. We are asked by some to abandon Genesis to science, salvation by redemption to anthropology, the life of the Spirit to psychology, the very Word itself to higher criticism.5
not denied My name
They had not denied Him by their words (Mtt. Mat. 10:32-33; Mat. 26:70-72; Luke Luke 12:8-9; 1Jn. 1Jn. 2:22-23) or through their actions (Pr. Pr. 30:9; Acts Acts 3:13-15; 1Ti. 1Ti. 5:8). In the midst of the ultimate test, the church at Pergamos held fast to His name and did not deny His faith. (See commentary on Revelation 2:13.) In the days of the Tribulation, many will deny His name by taking the name of another (John John 5:43; Rev. Rev. 13:17+; Rev. 14:11+).
3 Richard Chenevix Trench, Commentary on the Epistles to the Seven Churches in Asia (Eugene, OR: Wipf and Stock Publishers, 1861).