There are many who argue that the new covenant fully established itself on the cross and the old covenant fully disappeared at the cross and criticize the teaching of a fading away period for the old covenant.
No. 1. The covenant did not disappear immediately.
By calling this covenant “new,” he has made the first one obsolete; and what is obsolete and outdated will soon disappear.
At the time of writing of Hebrews i.e. AD63 “it will soon disappear”.
2. You will see the writer of Hebrews saying that while the old temple was standing the Jews were not reading to enter the real temple which is themselves.
The Holy Spirit was showing by this that the way into the Most Holy Place had not yet been disclosed as long as the first tabernacle was still functioning. This is an illustration for the present time, indicating that the gifts and sacrifices being offered were not able to clear the conscience of the worshiper.
3. The Jews had not understood that the real promised land was actually their own bodies. That’s why they had to wait (even at the time of the writing of Hebrews which was after the cross) for the Jews of that generation to inherit that kingdom.
These were all commended for their faith, yet none of them received what had been promised,
Notice the faith Heroes had to wait for the Jews in that generation?
Therefore, since we are receiving a kingdom that cannot be shaken, let us be thankful, and so worship God acceptably with reverence and awe,
Note it is at that time they are receiving the kingdom. They had yet to receive it.
4. Also the sacrifices were going to be abolished. Not abolished yet.
but only a fearful expectation of judgment and of raging fire that will consume the enemies of God.
The fire and the judgment here are referring to the destruction of the temple will take place in 7 years time from the time of writing.
5. Jesus said that it will happen within a generation. It took Moses a whole generation to enter the promise land. So it did not take place immediately.
This is clearly taught in Hebrews 4.
A Sabbath-Rest for the People of God
Therefore, since the promise of entering his rest still stands, let us be careful that none of you be found to have fallen short of it.
At that time the time still stood.
Therefore since it still remains for some to enter that rest, and since those who formerly had the good news proclaimed to them did not go in because of their disobedience,
God again set a certain day, calling it “Today.” This he did when a long time later he spoke through David, as in the passage already quoted:
“Today, if you hear his voice,
do not harden your hearts.”
That word “today” spoke about the last days of the old covenant and the beginning of the new.
Let us, therefore, make every effort to enter that rest, so that no one will perish by following their example of disobedience.
If they had already fully entered the covenant why is there is a struggle to enter that rest?
This period where the old and new existed at the same time is called the Last Days or the Final Days.