The answer to this question will touch many believers in ways that they do not want to ever think about. In ways that will challenge their theology and even their core doctrines (which are all man-made by the way and should never be deemed to be 'unquestionable'). With that in mind, I pray this is not received as an attack against any individual or church group.
To start, the letters most likely did have specific intent and meaning to be directed at those specific churches which existed at the end of the first century. However, Revelation was given to ALL churches and ALL believers as the ONLY prophecy book for the Church Age. With the prophecies to the Jewish people now on hold until after the Church Age is concluded, Revelation is necessary and I would argue REQUIRED of God to present to us (Amos 3:7). With Revelation divided into sections we can see that Amillennialists will primarily agree that almost ALL of Rev. was completed BEFORE it was written, but that would mean there is no prophecy for the Church Age and that would make God a liar, so, I am going out on a limb and declaring that way of thinking to be a False teaching. That leaves Premillennialists. The majority of these will insist that the entirety (at least from ch.4 on) of Rev. is reserved for AFTER the Church Age is completed, thus leaving ONLY the 7 letters as actual prophecy for the Church Age.
Rev 1:1 says this prophecy was given by Jesus and that these things "must SHORTLY come to pass" or 'from this time onward'.
Rev 1:4 mentions "seven spirits which are before his throne". (Rev 5:6 - They are sent from God out into the whole earth) Something I stumbled across in Gen 21:25-32 talks about Abraham making a covenant with Abimelech after a dispute over a well. Abimelechs servants are described as "violently taking AWAY the well". (How do you take away a well?) Anyway, Abraham GAVE 7 of his Sheep and oxen to Abimelech even though Abraham was the victim (?), and entered into a covenant with him. Now, the 7 sheep were "sent off by themselves" so that "they may be a witness unto me, that I have digged this well". The parallels with Jesus are astounding including the fact that Jesus began His ministry at a well and speaking to a Non-Jew.
Rev 1:6 says that Jesus (at this point) has been given "GLORY" and "DOMINION". Subsequent chapters show that Jesus is given more and more of these "special honors" until the end when ALL the honors are bestowed. This suggests that these honors are increasing at various points along the prophetic time line. This also helps us to apply those situations and surrounding events to the proper corresponding visions and likewise will tell us if we are applying a vision to the WRONG prophetic event on that time line. For example, Rev 5:12 describes ALL the honors(of which there are 7) that Jesus is "worthy" to receive, but it is clear that He has NOT yet received them at this point. Vs 13 says He has received at this time only 4 of those 7. Riches, Wisdom, and Strength have yet to be given.
Rev 1:12 mentions 7 golden candlesticks (Rev 1:20 - the 7 churches). Candlesticks or lamps are used to "HOLD the LIGHT" = The word of truth/ the gospel message. This idea can be also applied to us as individuals.
Rev. 1:16 says He has 7 stars in His right hand (the angels/messengers of the 7 churches - Rev. 1:20)and a sharp two edged sword coming out of His mouth (the Word).
Rev 1:18 specifies to John what he must do... "write the things he has seen, the things which are, and the things which will be hereafter".
To the letters we see both good and bad things said about the first 5 including warnings for NOT repenting or changing their ways and rewards for doing what is right. It is not until the 4th (Thyatira) that obvious parallels with the dominant church of the Middle Ages - Catholicism - arise to give a basis to proceed forward and backward from. We are told the later works are actually greater than at first which is something good and can be seen in the world that the Catholic church is indeed doing great and wonderful acts of giving. However, the bad seems to be far worse than all the others as forms of idolatry are prevalent and even promoted. Rev 2:22 says those who do not repent WILL be cast into great tribulation even to the point of killing them for the reason of showing all "believers" that He will judge them according to their works (see Ch. 17). Then vs 26 states that those who "overcome" will have power over the nations although being ruled over with a "rod of iron" (no tolerance for evil). This is a reference to the 1000 years and so I conclude that, just like in the other churches, some may be "snatched/raptured" while others are left-behind. This is also the first time that the phrase about "having an ear" is at the END of individual prophecy. I believe this portrays the first three churches to be essentially phased out of existence by the time the tribulation begins.
Sardis then can rightly be appointed to the "reformed" churches which may even be implicated as "daughters" of the mother church (Catholicism). The message depicts a dying church with fewer and fewer people that "have not defiled their garments". Rev 3:5 mentions the reward of a white garment AND not being blotted out of the book of life AND Jesus will even confess their name before God and his angels. (The 5th seal mentions impatient souls in heaven waiting for something but then are given white robes and told to wait a little while longer).
The final two churches depict the raptured and the left-behind believers (the FINAL living Church - body of Christ as a whole when the rapture occurs). The first (Philadelphia) has nothing bad said against it and in fact is promised to be "kept from the hour of temptation" and even made a permanent part of the temple in heaven. The second has NOTHING good said about it, except that they still have a chance to repent and then also "go into" the 1000 year reign (have access to the throne to sup with him)