The margin says shortly means "quickly" or "swiftly." This must be understood in terms of what the book of Revelation was designed to reveal. Verse 10 tells us that the book was designed for the Day of the Lord. "Shortly" has to be seen in light of verse 10.
When was the apostle John on the island of Patmos? All indications are that he received this vision somewhere in the AD 90s—somewhere between AD 90 and 100. The Temple in Jerusalem had already been destroyed by the Romans under Titus.
Think about this word "shortly" in reference to the time in which the prophecy was given. Did Jesus Christ mean shortly after He gave it? What happened historically quickly or shortly after Christ gave this prophecy to the apostle John? Nothing. Nothing happened. By and large, almost 20 centuries later, very little in Revelation has yet happened.
What does this say about the design of the book of Revelation? It says that its primary intent is for the time we are living in right now! When Christ said that these things must shortly come to pass, what He meant was that once the things in Revelation begin to occur, they will happen very quickly in historical terms. They will begin unfolding so fast, it will take our breath away!
To whom was this book written? Verse 11 says that it was written to the churches in Ephesus, Smyrna, Pergamos, Thyatira, Sardis, Philadelphia, and Laodicea. If the book was written to the end-time people, then we have to conclude that the message as delivered to the actual churches in Asia Minor was only secondary. The attitudes, the conduct, the events occuring in those seven congregations were only models of what was going to happen later.
However, Ephesus, Smyrna, Pergamos, Thyatira, Sardis, Philadelphia, and Laodicea are somewhere extant on earth today. Not just in the form of attitudes, but maybe also in the form of true-church organizations. All seven churches must be in existence at the end. This only makes sense because the book is concerned primarily with the end-time.
Does it not seem reasonable and logical that, if Christ wanted to get a message to each of the churches, and He only had a moment to spend with each one, He would be extremely selective in what He had to say? He would carefully design His message to contain the nucleus of what He wanted to get across. It would be quick, concise, and hit the nail right on the head. What He said would be of the utmost importance to them in regard to their responsibilities at the end time.
If His church were to be in existence at the end—and surely it is because He says that the gates of the grave would not prevail against it (Matthew 16:18)—then He would give His church what it needed most of all to survive and endure that period of time. He would not waste His opportunity speaking on trivial matters!
He would get to things that are essential to His people to get them through the trouble and into the Kingdom with as much growth as possible! This is the essence of those seven messages. It is the reason why they were written.