In a nutshell:
The Gospel is that the Kingdom of God has arrived amongst men, who must enter its Rest by force, throwing away unbelief.
John the Baptist believed the Messiah would come and cleanse Israel, by fulfilling the law, following it perfectly. All the blessings of Deuteronomy 29 would follow and the world would be saved by becoming Jewish.
Jesus does indeed fulfill the law , but Jesus is also the Bridegroom, He is the both the one who obeys AND the one who rewards obedience with blessings. He is also the one who makes a new Covenant with the Household of God, after completing the old contract. Now they, Jews, must be saved by becoming New Covenanters, by entering the Body of Christ. By doing so, they will enter rest, and receive the promise made to Abraham that through them the nations of the world will be blessed.
If the Good News is that the blessing to the People of God, the giving of Kingdom of God, has started through perfect obedience as promised by Deuteronomy 29, then the resurrection of Christ is its greatest proof, the feast spread in the desert, the Wedding Banquet:
Luke 11
20“But if I cast out demons by the finger of God, then the kingdom of God has come upon you.
1 Corinthians 15
1Now, brothers and sisters, I want to remind you of the gospel I preached to you, which you received and on which you have taken your stand.
2By this gospel you are saved, if you hold firmly to the word I preached to you. Otherwise, you have believed in vain.
3For what I received I passed on to you as of first importance
a : that Christ died for our sins according to the Scriptures,
4that he was buried, that he was raised on the third day according to the Scriptures,
5and that he appeared to Cephas,
b and then to the Twelve.
6After that, he appeared to more than five hundred of the brothers and sisters at the same time, most of whom are still living, though some have fallen asleep.
7Then he appeared to James, then to all the apostles,
8and last of all he appeared to me also, as to one abnormally born.
9For I am the least of the apostles, and not fit to be called an apostle, because I persecuted the church of God.
10But by the grace of God I am what I am, and His grace toward me did not prove vain; but I labored even more than all of them, yet not I, but the grace of God with me.
11Whether then
it was I or they, so we preach and so you believed.
12Now if Christ is preached, that He has been raised from the dead, how do some among you say that there is no resurrection of the dead?
13But if there is no resurrection of the dead, not even Christ has been raised;
14and if Christ has not been raised, then our preaching is vain, your faith also is vain.
15Moreover we are even found
to be false witnesses of God, because we testified against God that He raised Christ, whom He did not raise, if in fact the dead are not raised.
16For if the dead are not raised, not even Christ has been raised; 17and if Christ has not been raised, your faith is worthless; you are still in your sins.
Hosea 13:14aShall I ransom them from the power of Sheol? Shall I redeem them from death?
O Death, where are your thorns? O Sheol, where is your sting?
Matthew 11
2Now when John, while imprisoned, heard of the works of Christ, he sent word by his disciples
3and said to Him, “Are You the Expected One, or shall we look for someone else?”
4Jesus answered and said to them, “Go and report to John what you hear and see:
5the BLIND RECEIVE SIGHT and the lame walk, the lepers are cleansed and the deaf hear,
the dead are raised up, and the POOR HAVE THE GOSPEL PREACHED TO THEM. 6“And blessed is he who does not take offense at Me.”
Matthew 11
"Woe to you, Chorazin! Woe to you, Bethsaida! For if the miracles had occurred in Tyre and Sidon which occurred in you, they would have repented long ago in sackcloth and ashes.
Psalms 78:11They forgot His deeds And His miracles that He had shown them.
Psalms 78:19Then they spoke against God; They said, "Can God prepare a table in the wilderness?
John 10:14“I am the good shepherd, and I know My own and My own know Me,
15even as the Father knows Me and I know the Father; and I lay down My life for the sheep.
16“I have other sheep, which are not of this fold; I must bring them also, and they will hear My voice; and they will become one flock
with one shepherd.
17“For this reason the Father loves Me, because I lay down My life so that I may take it again. 18“No one has taken it away from Me, but I lay it down on My own initiative. I have authority to lay it down, and I have authority to take it up again. This commandment I received from My Father.”
Salvation is by repenting, meta noia, changing mind, stopping serving mammon, starting serving God.
For the strong, persistent:
Luke 18:
8“I tell you that He will bring about justice for them quickly. However, when the Son of Man comes, will He find faith on the earth?”