If you look at the seven churches in these two chapters you can imagine they are in the form of a circuit, with the whole book being passed on from church to church. So even though the content is directed specifically to these seven representative churches, both all these and other churches and even those who read this today will be blessed to read them all.
Revelation 1:3 Blessed is he that readeth, and they that hear the words of this prophecy, and keep those things which are written therein: for the time is at hand.
Rev 3:6 He that hath an ear, let him hear what the Spirit saith unto the churches.
Each of these sections which I quote below refer again back to the initial vision identifying who is speaking this revelation, and the promise give to those who hear and overcometh.
Rev 3:1 And unto the angel of the church in Sardis write; These things saith he that hath the seven Spirits of God, and the seven stars; I know thy works, that thou hast a name that thou livest, and art dead.
Rev 3:5 He that overcometh, the same shall be clothed in white raiment; and I will not blot out his name out of the book of life, but I will confess his name before my Father, and before his angels.
Rev 3:7 And to the angel of the church in Philadelphia write; These things saith he that is holy, he that is true, he that hath the key of David, he that openeth, and no man shutteth; and shutteth, and no man openeth;
Rev 3:12 Him that overcometh will I make a pillar in the temple of my God, and he shall go no more out: and I will write upon him the name of my God, and the name of the city of my God, which is new Jerusalem, which cometh down out of heaven from my God: and I will write upon him my new name.
Rev 3:14 And unto the angel of the church of the Laodiceans write; These things saith the Amen, the faithful and true witness, the beginning of the creation of God;
Rev 3:21 To him that overcometh will I grant to sit with me in my throne, even as I also overcame, and am set down with my Father in his throne.
Going back to Chapter 2 these are the promise verses to the four other churches.
Rev 2:7 He that hath an ear, let him hear what the Spirit saith unto the churches; To him that overcometh will I give to eat of the tree of life, which is in the midst of the paradise of God.
Rev 2:10 Fear none of those things which thou shalt suffer: behold, the devil shall cast some of you into prison, that ye may be tried; and ye shall have tribulation ten days: be thou faithful unto death, and I will give thee a crown of life.
Rev 2:17 He that hath an ear, let him hear what the Spirit saith unto the churches; To him that overcometh will I give to eat of the hidden manna, and will give him a white stone, and in the stone a new name written, which no man knoweth saving he that receiveth it.
Rev 2:26 And he that overcometh, and keepeth my works unto the end, to him will I give power over the nations:
Rev 2:27 And he shall rule them with a rod of iron; as the vessels of a potter shall they be broken to shivers: even as I received of my Father.
Rev 2:28 And I will give him the morning star.
Finally this familiar verse is often interpreted as a call for individual acceptance of Jesus into our lives.
Rev 3:20 Behold, I stand at the door, and knock: if any man hear my voice, and open the door, I will come in to him, and will sup with him, and he with me.
I have heard it also interpreted that this is more of a verse of pending judgment..
James 5:9 Grudge not one against another, brethren, lest ye be condemned: behold, the judge standeth before the door.
Or that the application if directed towards the church as a whole and whether when Christ comes he will actually find truth faith even in his churches, or will they not open the door to him.
Luke 18:8 I tell you that he will avenge them speedily. Nevertheless when the Son of man cometh, shall he find faith on the earth?
John Gill speaks to the verse this way:
Revelation 3:20 Gill
Behold, I stand at the door and knock,…. The phrase of standing at the door may be expressive of the near approach, or sudden coming of Christ to judgment, see Jas 5:9; and his knocking may signify the notice that will be given of it, by some of the immediate forerunners and signs of his coming; which yet will be observed but by a few, such a general sleepiness will have seized all professors of religion; and particularly may intend the midnight cry, which will, in its issue, rouse them all:
if any man hear my voice; in the appearances of things and providences in the world:
and open the door; or show a readiness for the coming of Christ, look and wait for it, and be like such that will receive him with a welcome:
I will come unto him, and sup with him, and he with me; to and among these will Christ appear when he comes in person; and these being like wise virgins, ready, having his grace in their hearts, and his righteousness upon them, he will take them at once into the marriage chamber, and shut the door upon the rest; when they shall enjoy a thousand years communion with him in person here on earth; when the Lamb on the throne will feed them with the fruit of the tree of life, and lead them to fountains of living water, and his tabernacle shall be among them.