The New Testament is the fulfillment of the Old Testament. But If your asking what is the main message that it teaches that would be the fulfillment of the Prophecies of the Old Testament and Jesus the Christ is Born to Save the Sinner from the Lies of Satan.
Which could of caused them to lose their Souls or their place within God's Kingdom. If He did Not Come. There would be no Heaven. Because Heaven is Wherever God resides.If He did not defeat Death who is Satan. All flesh would have died along with the spiritual body and possibly the soul as well.
Or the Salvation Gospels which many know them as, but many do not understand them or have ever even read them for themselves. God says His children are destroyed for lack of Knowledge. But is this only a physical death or is this the spiritual Death? For we have two bodies and a soul, is your's at Risk and how do you know?
Well that is what the New Testament reveals to the true believer in Christ The Lord Jesus and what He did within the New testament.