(2 Peter 3:15 KJV) And account that the longsuffering of our Lord is salvation; even as our beloved brother Paul also according to the wisdom given unto him hath written unto you;
Peter is telling the Galatians that Paul had previously written to them.
Yet, heir claims that the Galatians Peter is writing to are being preached a different gospel than Paul preached, despite Peter clearly telling the Galatians that Paul has previously written to them.
15. And account that
the longsuffering of our Lord is salvation; even
as our beloved brother Paul
also according to the wisdom given unto him hath written unto you;
In other words,
16.
As also in all his epistles,
speaking in them of these things; in which are some things hard to be understood, which they that are unlearned and unstable wrest, as they do also the other scriptures, unto their own destruction.
What things? The Lord's delayed return in wrath, in longsuffering.
In other words, this - Acts 3:
18. But those things, which God before had shewed by the mouth of all his prophets, that Christ should suffer, he hath so fulfilled.
19. Repent ye therefore, and be converted, that your sins may be blotted out, when the times of refreshing shall come from the presence of the Lord.
20. And he shall send Jesus Christ, which before was preached unto you:
21.
Whom the heaven must receive until the times of restitution of all things, which God hath spoken by the mouth of all his holy prophets since the world began.
22. For Moses truly said unto the fathers, A prophet shall the Lord your God raise up unto you of your brethren, like unto me; him shall ye hear in all things whatsoever he shall say unto you.
23.
And it shall come to pass, that every soul, which will not hear that prophet, shall be destroyed from among the people.
Was interrupted by this - Romans 9:
22. What if God, willing to shew his wrath, and to make his power known, endured with much longsuffering the vessels of wrath fitted to destruction:
23. And that he might make known the riches of his glory on the vessels of mercy, which he had afore prepared unto glory,
24. Even us, whom he hath called, not of the Jews only, but also of the Gentiles?
1 Timothy 1:
15. This is a faithful saying, and worthy of all acceptation, that Christ Jesus came into the world to save sinners; of whom I am chief.
16. Howbeit for this cause I obtained mercy,
that in me first Jesus Christ might shew forth all longsuffering,
for a pattern to them which should hereafter believe on him to life everlasting.
Romans 11:
25. For I would not, brethren, that ye should be ignorant of this mystery, lest ye should be wise in your own conceits; that blindness in part is happened to Israel, until the fulness of the Gentiles
be come in.
26. And so all Israel shall be saved: as it is written, There shall come out of Sion the Deliverer, and shall turn away ungodliness from Jacob:
27. For this is my covenant unto them, when I shall take away their sins.
28. As concerning the gospel, they are enemies for your sakes: but as touching the election, they are beloved for the father’s sakes.
That, interrupted this - Romans 15:
8. Now I say that Jesus Christ was a minister of the circumcision for the truth of God, to confirm the promises made unto the fathers:
9. And that the Gentiles might glorify God for his mercy; as it is written, For this cause I will confess to thee among the Gentiles, and sing unto thy name.
10. And again he saith, Rejoice, ye Gentiles, with his people.
11. And again, Praise the Lord, all ye Gentiles; and laud him, all ye people.
12. And again, Esaias saith, There shall be a root of Jesse, and he that shall rise to reign over the Gentiles; in him shall the Gentiles trust.
And as a result...
13. Now the God of hope fill you with all joy and peace in believing, that ye may abound in hope, through the power of the Holy Ghost.
14. And I myself also am persuaded of you, my brethren, that ye also are full of goodness, filled with all knowledge, able also to admonish one another.
15. Nevertheless, brethren, I have written the more boldly unto you in some sort, as putting you in mind, because of the grace that is given to me of God,
16. That I should be the minister of Jesus Christ to the Gentiles, ministering the gospel of God, that the offering up of the Gentiles might be acceptable, being sanctified by the Holy Ghost.
17. I have therefore whereof I may glory through Jesus Christ in those things which pertain to God.
18. For I will not dare to speak of any of those things which Christ hath not wrought by me, to make the Gentiles obedient, by word and deed,
19. Through mighty signs and wonders, by the power of the Spirit of God; so that from Jerusalem, and round about unto Illyricum, I have fully preached the gospel of Christ.
20. Yea, so have I strived to preach the gospel, not where Christ was named, lest I should build upon another man’s foundation:
21. But as it is written, To whom he was not spoken of, they shall see: and they that have not heard shall understand.
What both their letters to the other had in common was that "whatsoever things were written aforetime were written for our learning, that we through patience and comfort of the scriptures might have hope," Romans 15:4.
1 Corinthians 9:
11. Now all these things happened unto them for ensamples: and they are written for our admonition, upon whom the ends of the world are come.
In this, you're hard pressed to show what letters Peter says Paul wrote "to" them.
Neither would throw out a part of the Scriptures.