Q. According to Hebrews 9, Jesus ministry was part of the Old Testament until His death on the cross. So in your opinion where does the New Testament begin?
A. I don’t interpret Hebrews 9 quite the same as you describe it, although from man’s point of view it’s reasonable to say that until Jesus had actually completed His work of redemption, the New Testament wasn’t officially in force. But consider this.
The Jewish name for the part of the Bible we call the Old Testament was The Law And The Prophets. Matt. 11:11-15 records these words of the Lord.
“I tell you the truth: Among those born of women there has not risen anyone greater than John the Baptist; yet he who is least in the kingdom of heaven is greater than he. From the days of John the Baptist until now, the kingdom of heaven has been forcefully advancing, and forceful men lay hold of it. For all the Prophets and the Law prophesied until John.”
To me this means Jesus was saying John the Baptist was the last of the Old Testament prophets and is not part of the Church. That means that the New Testament effectively began when John’s ministry ended.