The apostle Peter, toward the end of his ministry, wrote a couple things that were poignant and relevant, regarding the scriptures.
The first was that these scriptures didn't come simply from the mind and heart of the man used by God to bring them to us:

II Peter 1:20 Knowing this first, that no prophecy of the scripture is of any private interpretation.
 21 For the prophecy came not in old time by the will of man: but holy men of God spake as they were moved by the Holy Ghost.

The Holy Ghost is the true author of the scriptures.
He inspired what those men wrote in the scriptures.
The doctrines and teachings came from the Holy Ghost to us through the scriptures, written by men as they were inspired by the Holy Ghost.
Then a little later in his same epistle, the apostle Peter mentions Paul's writings:

II Peter 3:15 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;
 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.

In line 16 Peter is showing that Paul's writings are scripture, as are the others scriptures: e.g. John,  Mark, James, Peter, and Luke.
So, a quote from one of Paul's epistles, is a quote from scriptures, a quote from the Holy Ghost.

Also, the question of Christ or Paul comes up in Pauls first letter to the church in Corinth:

I Corinthians 1:11 For it hath been declared unto me of you, my brethren, by them which are of the house of Chloe, that there are contentions among you.
 12 Now this I say, that every one of you saith, I am of Paul; and I of Apollos; and I of Cephas; and I of Christ.
 13 Is Christ divided? was Paul crucified for you? or were ye baptized in the name of Paul?
 14 I thank God that I baptized none of you, but Crispus and Gaius;
 15 Lest any should say that I had baptized in mine own name.
 16 And I baptized also the household of Stephanas: besides, I know not whether I baptized any other.
 17 For Christ sent me not to baptize, but to preach the gospel: not with wisdom of words, lest the cross of Christ should be made of none effect.

To the question of Christ or Paul, Paul answered that, it is Christ.