Specifically, John wrote to the seven churches of God, which were in Asia (Revelation 1:4), part of the same area to which Peter sent his epistles (1Peter 1:1). The Apostle Paul also wrote letters to churches in seven places: Rome, Corinth, Galatia, Ephesus, Philippi, Colossae, and Thessalonica. The number seven is used 54 times in 31 verses in this book. It is seven churches, seven spirits, seven horns, seven eyes, seven seals etc. Why seven? Many scholars would conclude it is the number of completeness, perhaps derived from the creation week of Genesis 1 & 2. It took God six days to create the universe and every living thing in it, but he didn’t stop there. He created a day of rest on the seventh day by resting on that day. That is, he ceased his labor on that day, not that God was weary and needed to relax. Later he would command that the seventh day would be used as a day of worship to commemorate the completed or whole creation of God (Exodus 20:8-11).
John’s address to the seven churches in Asia is similar to how Peter and Paul addressed their hearers in their letters (cf. 1Peter 1:1-2; Romans 1:1, 7; 1Corinthians 1:1-3; 2Corinthians 1:1-2 etc.). Nearly all of the authors of the New Testament began their works with sending grace and peace to their readers from God (the Father) and Jesus. The only exceptions to this rule are: Hebrews, James and 1&3 John, but even Hebrews and 3John close with something similar (Hebrews 13:20-21, 25; 3John 1:14)
Therefore, the phrase “He who is, who was and is to come (or is coming)” seems to point to God, our Father. If we take into consideration how each and every epistle is addressed (i.e. “grace and peace” coming from God and Jesus) John seems to say the same here. Grace and peace from “He who is, who was and is to come” (God, the Father – Revelation 1:4) and from Jesus (Revelation 1:5).
The phrase “is to come” or “is coming” simply means John is speaking of the eternal God. He is, he was (or has always been) and he is now, and he is to come (or always will be). Simply put, this is how the Jews of John’s day would have understood the phrase. Note how the ancient rabbis wrote of it:
“Rabbi Jose said, By the name Tetragrammaton, (i.e. YEHOVAH,) the higher and lower regions, the heavens, the earth, and all they contain, were perfected; and they are all before him reputed as nothing:- vehu hayah, vehu hoveh, vehu yihyeh; and HE WAS, and HE IS, and HE WILL BE.” [Sohar Chadash, fol. 7, 1]
“The holy blessed God said to Moses, tell them:- ani shehayithi, veani hu achshaiu, veani hu laathid labo; I WAS, I NOW AM, and I WILL BE IN FUTURE.” [Shemoth Rabba, sec. 3, fol. 105, 2]
The Seven Spirits, concerning whom John writes in Revelation 1:4; 3:1; 4:5 and 5:6, seem to be a mystery, because he never defines who or what the Seven Spirits are. Many scholars have assumed it is a reference to the Holy Spirit, but this is a purely subjective analysis. Personally, I find the interpretation impossible for two reasons. First, the Holy Spirit never sends greetings throughout any of the other writings of the New Testament, and, secondly, it contradicts the understanding that the Holy Spirit was with John’s intended readers at all times. If, therefore, the Holy Spirit was always with believers in the first century AD, why would he send greetings?
Another interpretation is that the seven spirits refer to seven angels, perhaps the seven angels of the seven churches to whom John addresses his writing (cf. Revelation 1:20). Nevertheless, this also seems inappropriate. First, it misses the mark, because no angel ever sent a greeting to any of the churches in the New Testament, so why would seven angels send their greeting here? Secondly, it is inappropriate, because the verb esti (G2076) is the third person **singular** and present indicative of the verb eimi (1510). It would be like saying seven cars **is** going down the road. It simply doesn’t fit.
Rather I believe it points to Jesus. That is “… and from the seven spirits… EVEN from Jesus Christ…” (end of verse-4 and beginning of verse-5). The phrase “and Jesus Christ” in Revelation 1:5 should be understood as explanatory, that is, the Seven Spirits EVEN Jesus Christ—as the HRB, LITV, MKJV translations have it. Revelation 3:1 tells us that Jesus ‘has’ the Seven Spirits of God, and Revelation 4:5 defines them as the seven lamps burning before the throne of God. On the other hand, Revelation 5:6 tells us that the Seven Spirits of God are the seven eyes of the Lamb (Jesus). Therefore, the seven lamps of Revelation 4:5 are the seven eyes of the Lamb (cf. Mark 6:22). So, what should we make of this picture? Notice what Isaiah wrote of Seven Spirits:
And there shall come forth a rod out of the stem of Jesse, and a Branch shall grow out of his roots: And the spirit of the LORD shall rest upon him, the spirit of wisdom and understanding, the spirit of counsel and might, the spirit of knowledge and of the fear of the LORD; (Isaiah 11:1-2)
It seems to me, therefore, that the Seven Spirits of Revelation 1:4 refer to Jesus, the Branch (in Isaiah) that grows out of the root of Jesse, David’s father. In other words, John sends grace and peace to the seven churches from God, our Father, and from Jesus, our Messiah.
 See New American Standard Exhaustive Concordance