Jesus told the messenger of the church at Philadelphia that the church had kept the word of Jesus (the Gospel) and have not denied his name (Revelation 3:8). That is, the church at Philadelphia had not compromised the word of God, but guarded its purity, preaching it and enduring the consequences for the sake of Jesus’ name. Therefore, Jesus said he had set before them an open door, and I believe he meant he had opened a door of understanding of the Gospel among the people in that city and perhaps the surrounding area.
After Paul left Corinth he ministered at Ephesus for three years. He wrote that a great and effective door was opened for him there, and he intended to stop there for awhile before leaving for Jerusalem (1Corinthians 16:8-9). However, just because a door was opened so he could preach the Gospel, that didn’t mean there were none in Ephesus who fought him and sought to undo his work there, because Paul told the Corinthians that he was opposed by many adversaries (verse-9). The fact is, wherever Paul went to preach the Gospel, when a door was opened to him, he was met with enemies of the faith, even enemies who sought his life. So, when the Lord opens a door to preach the Gospel, it nearly always means trouble comes with the open door, because, although Jesus is King of kings and Lord of lords, the nations are still angry that he has taken the reigns to rule (Revelation 11:15, 18).
Moreover, when it became clear in a dream that the Lord opened a door for the Gospel to go to Europe, Paul sailed to Macedonia (2Corinthians 2:12-13). It is God who causes us to be victorious over the nations that oppose us (2Corinthians 2:14; cf. Revelation 11:18). Paul saw his life as a sacrifice to the Lord (cf. 2Timotyh 4:6; Philippians 2:17), giving off a scent to both believer and unbeliever. To the one he was a scent of Christ showing nothing but death, but to the other he was the scent of Christ having the odor and expectation of life (2Corinthians 2:14-16). Similarly, the church in Philadelphia, guarding the purity of the word of God, preached the Gospel of Christ, giving off the odor of Christ to both those willing to embrace the good news and to those who rejected it. It was the same odor, the same Gospel, but to the one it meant life, while to the other it meant death. So, with the Gospel came trouble, perhaps not death, but trouble nonetheless. Yet, the church embraced its responsibility without denying the name of Christ.
While later the nations would be angry with the spread of the Gospel, and even persecuted those who spread the good news, they were powerless to prevent folks from believing with their hearts. Men may be able to govern the behavior of other men, perhaps even force people to behave as their governors wish, but these same authorities are powerless to govern the hearts of men. Once the door of the Gospel is open, no man is able to shut that door. Once a man sees a thing, no one is able to command him to un-see it. Once he hears a thing, no one is able to demand that he un-hear it. It is done, and no one has the power or authority to undo the matter. This is the victory of the Gospel. Once it is preached, it cannot be un-preached. Folks who have heard and believed, now have a choice. They may value life and obey the Lord, or fear man and continue in their rebellion, but they cannot escape what they have heard and believed.
When Jesus told the church it had only a little strength, Jesus was probably referring to the power and strength the world respects, because not many wise men in the affairs of this world and not many mighty men nor men of nobility have been chosen by God to preach the Gospel (1Corinthians 1:26). Rather, God has chosen men such as this messenger and other believers in Philadelphia in order that the wise would be ashamed, and that the mighty men would be disgraced (1Corinthians 1:27), and that the nobility would see the weakness of their own influence and power in the affairs of God (1Corinthians 1:28)
Instead, the weak were chosen to be victorious over the wise, the mighty and the nobility of this world in order that the power of God could be understood in its perfect or true sense (2Corinthians 12:9). Therefore, the church in Philadelphia could take heart in the face of powerful opposition, knowing that the power of God rested upon its own weak status. Moreover, they could glory in their being reproached, in necessity, in persecution and in distress, because all any believer was able to do, though weak in this world’s power, was done for the sake of Christ, and in so doing victory was gained (2Corinthians 12:9-10).
 The pronouns in the Greek are in the singular, and at first one might assume the messenger alone is being addressed, and, therefore, at least at this point Jesus couldn’t be addressing the whole church, but this isn’t so. Jesus always ends each letter with the phrase: “he that has an ear to hear, let him hear what the Spirit says to the churches” (Revelation 2:7, 11, 17, 29; 3:6, 13, 22). Therefore, the singular pronouns refer to each individual church as one entity.
 See Acts 9:23-25, 29-30; 13:4-8, 49-50; 14:1-7, 19-20; 15:1-2; 16:19-24; 17:5-9, 13; 18:12-13; 19:21-41; 20:3; etc.