When did the New Testament begin?
Hebrews 9:1 speaks of a "first covenant", and
Hebrews 9:15 speaks of a "new covenant".
For where a testament is,
there must also of necessity be the death of the testator.
For a testament is of force after men are dead:
otherwise it is of no strength at all while the testator liveth.
Hebrews 9:16-17
This means that the New Testament was not in effect until
Jesus died. Jesus died in the Gospels in
Matthew 27:50,
Mark 15:37,
Luke 23:46, and
John 19:30.
Here is a sketch to illustrate this point: