The Old Testament of the Bible presents God’s original way of interacting with his people (a covenant). It consists of a list of rules and expectations (the law).
The New Testament of the Bible introduces Jesus and a new way to interact with God. Jesus sets aside the old rules and asks people to believe in him (a new covenant).
When does this transition from the old way to the new way occur? It’s simplistic to say the Bible’s two Testaments represent the demarcation. I always thought the switch started in earnest when Jesus died and was completed when he resurrected.
However, Jesus says the old way of doing things applied until John the Baptist began baptizing people and telling them to get ready. This implies the new way begins when John points people to Jesus. Perhaps John’s preaching is the pivotal point between the old and new ways of doing things.
Regardless of when the transition occurred, it has happened and Jesus provides a new way to connect with God.