Mr. Armstrong used to say that he thought the Bible was primarily written for the Philadelphia era of the Church of God. He based that on his understanding that until modern times the Bible was not both complete and widely available, and in Mr. Armstrong's day only the Philadelphia era of the Church of God had a deep and extensive understanding of the Bible.
The Philadelphia era of God's Church may be one of the most important audiences of the Bible but the widest audience will be people in the millennium and white throne judgment periods. The Bible will no doubt be the most read book by billions of people, and they will be able to understand it.
The nations will observe God's annual feasts and holy days during the millennium and white throne judgment periods. They will understand the meaning of those days, not only from our instruction, but from the Bible itself, just as we do.
The Feast of Trumpets, which we recently observed, is called a memorial of the blowing of trumpets (Leviticus 23:23-25). It is to be observed in memory of an event. But what event? We in the Church have no memory of a past event involving trumpets. Ancient Israel could perhaps remember their wanderings in the wilderness and various occasions when trumpets were blown.
When God says it is a memorial for the people, I think He primarily has something else in mind.
For literally billions of people who experience the seven last trumpet plagues, both those who die in those plagues and come up in the white throne judgment and those who survive into the millennium, the Day of Trumpets will be a memorial of something that happened in their lifetimes. They will remember it. It will be something very personal for them.
No doubt each of the seven trumpet blasts will be prolonged and extremely loud for maximum impact. All earth's atmosphere will probably resonate like a bell with the trumpet sound. Perhaps the sound wave will be what audio engineers call a "standing wave". There will be no escape from the sound. It will be terrifying.
All the first generation that lives into the millennium will remember hearing those trumpet blasts, as well as billions in the white throne judgment who died in the disasters those trumpet blasts signaled.
For them, the Day of Trumpets will be a memorial, a remembrance, of one of the most awesome events they ever experienced.
And the children of the first generation in the millennium will learn about those trumpet blasts, not only from the Bible and our teaching, but from their parents and grandparents. The knowledge of that experience will be passed on from generation to generation. They will observe Trumpets as we observe Passover and Pentecost, in remembrance of something that has happened in the past.
When they read these words in their Bibles, "In the seventh month, on the first day of the month, you shall have a sabbath-rest, a memorial of blowing of trumpets" (Leviticus 23:24), they will have something greater to remember than Israel being called to marching order in the wilderness to set out on their journeys.
Here are links to related chapters or sections in Preaching the Gospel:
The Day of Trumpets - the Second Coming of Christ, Chapter 2
The Feast of Tabernacles - the Millennial Rule of Christ, Chapter 2
The Last Great Day - the White Throne Judgment, Chapter 2