What is the main thing of the New Testament?

The main thing that the New Testament teaches us is how to have a relationship with Jesus Christ.

Another View:

The core message of the New Testament was Jesus' telling us to 'Repent' as the Kingdom of God is coming and that is why He established the Church of God to prepare all God calls (see John 6:44, 65) to become 'firstfruits' of the 'little flock.'