Heir nailed it. Resurrection was the point here.
PAUL SAYS this is the gospel that saved them...it is the same...Whether, then, it is I or they, this is what
we preach, and this is what you believed.
But if you want to prove her wrong, it's simple: show where anyone before Paul preached what he preached:
(a) both Jews and Gentiles could now be saved on the same basis of grace through faith
John 12:32 And I, when I am lifted up from the earth, will draw
all people to myself."
John 1:29 The next day John saw Jesus coming toward him and said, "Look, the Lamb of God, who takes away the sin
of the world!
(b) without any works to get or stay saved
Paul does not preach NO OBEDIENCE. Paul preaches that Jesus is the Lamb and no one has to do any ceremonial/purification works anymore.
As for no works, Jesus preached he will give rest.
Matthew 11:28 "Come to me, all you who are weary and burdened, and I will give you rest.
John 1:29 The next day John saw Jesus coming toward him and said, "Look, the Lamb of God, who takes away the sin
of the world!
Think about it...the LAMB OF GOD...the lambs sacrificed day after day for the ceremonial/purification works.
(c) completely apart from national Israel
Even Paul tells you first to the Jews then to the Gentiles.
Romans 1:16 For I am not ashamed of the gospel, because it is the power of God that brings salvation to everyone who believes: first to the Jew, then to the Gentile.
Romans 2:9
There will be trouble and distress for every human being who does evil: first for the Jew, then for the Gentile;
Read this scripture, for even Peter knew it was a gospel first for the Jews then to the Gentiles.
Acts 3:26 When God raised up his servant, he sent him first to you to bless you by turning each of you from your wicked ways."
(d) with Jews and Gentiles becoming co-equal members of Christ's Body
(e) who are impossible to ever be separated from Him.
Show all that being preached before Paul preached it, and you win.
Acts 10:43 All the prophets testify about him that
everyone who believes in him receives forgiveness of sins through his name."
Luke 24:46 He told them, “This is what is written: The Messiah will suffer and rise from the dead on the third day, 47
and repentance for the forgiveness of sins will be preached in his name to all nations,
John 4:42 They said to the woman, “We no longer believe just because of what you said; now we have heard for ourselves, and we know that this man really is the
Savior of the world.”