The lesson chart sets out four elements with which the Hebrew writer is concerned, namely the law, the priesthood, the throne, and the oath. His argument begins in the patriarchal era, the time of the early fathers Abraham, Isaac, and Jacob.
Contemporary with Abraham was Melchizedek who is named in the oath made to Christ. Abraham and Melchizedek are compared, and Melchizedek is found to be superior (Hebrews 7).
Therefore Christ’s priesthood is superior having been designated By God as "according to the order of Melchizedek".
The law, priesthood, and throne which came from Abraham, are but symbols and shadows of the true law, priesthood, and throne which are heavenly and eternal. The oath, which uses a different symbol and shadow (Melchizedek), confirms this.
When Christ came, and died on the cross he was offering himself as a perfect sacrifice once and for all. He then ascended into heaven, to become the new lawgiver, priest, and king.
The old symbols and shadows had no further purpose because the oath was fulfilled.
The real eternal kingdom was inaugurated in heaven, and although we are yet on earth we can belong to it and have Christ as our High Priest and King.