[BlueDragon]

Where does it say that the Church MUST NOT teach anything not in it?

The Apostle Paul writes;

I am amazed that you are so quickly deserting Him who called you by the grace of Christ, for a different gospel; which is really not another; only there are some who are disturbing you, and want to distort the gospel of Christ. But even though we, or an angel from heaven, should preach to you a gospel contrary to that which we have preached to you, let him be anathema. As we have said before, so I say again now, if any man is preaching to you a gospel contrary to that which you received, let him be accursed (Gal. 1:6-9).

Cyril of Jerusalem (A.D. 315-386);

Concerning the divine and sacred Mysteries of the Faith, we ought not to deliver even the most casual remark without the Holy Scriptures; nor be drawn aside by mere probabilities and the artifices of argument. Do not then believe me because I tell thee of these things, unless thou receive from the Holy Scriptures the proof of what is set forth: for this salvation, which is our faith, is not by ingenious reasonings, but by proof from the Holy Scriptures....In these articles we comprehend the whole doctrine of faith. For the articles of the Faith were not composed at the good pleasure of men, but the most important points chosen from all Scriptures, make up the one teaching of the Faith. This Faith, in a few words, hath enfolded in its bosom the whole knowledge of godliness contained both in the Old and New Testaments. Behold, therefore, brethren and hold the traditions (2 Thes. 2:15) which ye now receive, and write them on the table of your hearts....Now heed not any ingenious views of mine; else thou mayest be misled; but unless thou receive the witness of the prophets concerning each matter, believe not what is spoken; unless thou learn from Holy Scripture....receive not the witness of man.

NPNF2, Vol. 7,Cyril of Jerusalem, Catechetical Lectures 4:17

[annalex]

a gospel contrary

I did not ask where heresy is forbidden. Of course when a charlatan like Luther comes and preaches whatever suits him, let him be anathema.

The question is, if the Church teaches some other thing never mentioned or described in the Holy Scripture, -- for example, what makes the Eucharist valid or whether cloning is allowed, -- where is that prohibited in the Holy Scripture?

[PeevedPatriot]

In Gal 1:6-9 does Paul confine his preaching to his written word? Doesn't seem to me that he does. 1 Thes 2:13 tells us that Paul's oral teaching is truly the word of God: "And we also thank God constantly for this, that when you received the word of God which you heard from us, you accepted it not as the word of men but as what it really is, the word of God, which is at work in you believers." In 2 Thes 2:15 he admonishes to "hold to the traditions which you were taught by us, either by word of mouth or by letter." Where does Paul confine his teaching only to his written word?

Cyril of Jerusalem taught doctrines often cited by nonCatholics as unbiblical (purgatory, the Mass, magisterium, etc). Was he a poor exegete to find basis for Catholic doctrine in scripture or was he not really a sola scriptura adherent as you suggest?