When one has understood at what point the New Covenant began, then a number of other things in the Bible become clearer. Hebrews 9 is one passage that shows when the New Covenant took effect.
Therefore He is the mediator of a new covenant, so that those who are called may receive the promised eternal inheritance, since a death has occurred that redeems them from the transgressions committed under the first covenant. For where a will is involved, the death of the one who made it must be established. For a will takes effect only at death, since it is not in force as long as the one who made it is alive. (Hebrews 9:15-17)
The term “mediator” means the one through whom an agreement or covenant is made. Moses was the mediator, with angels, through whom God gave the First Covenant (Exodus 24:2-3; Galatians 3:19). Christ is the mediator of the New Covenant.
The writer of Hebrews makes a point from his word for “covenant”, which in New Testament Greek is diatheke. It can mean an agreement (covenant, contract), and also a will (testament) whereby a person leaves his estate to his heirs. Using the second meaning, Hebrews points out that while the man who has a will remains alive, his will is not in force. The will only takes effect when he has died.
The modern picture might be like this: While the man is living, the document on which his will is written is sealed up and kept safely in the office of his lawyers. Even though its contents may be known, its requirements and promises have no power yet. Then the man dies. His heirs come to the office of the lawyer. The will is brought out, opened and read to all the heirs. Because the man has died, his will is now in effect. His wealth is to be given according to the terms and conditions stated in the will. If the heirs wish to receive their share of what the man left them, they must abide by the terms of the will.