You’ve taken the words “what is written” out of context. Paul is referring to the OT warnings about boasting quoted earlier in the letter - 1:19, 31; 3:19-20. Furthermore, Paul writes elsewhere that the oral teachings of the apostles are on a par with the written word - 1Thess 2:13; 2Thess 2:15, 3:6.
It seems there is more to salvation that merely faith:
“And the dragon was enraged with the woman, and he went to make war with the rest of her offspring, who keep the commandments of God
AND have the testimony of Jesus Christ.” (Rev 12:14
”Here is the patience of the saints; those who keep the commandments of God
AND the faith of Jesus.” (Rev 14:12)
”What
does it profit, my brethren, if someone says he has faith but does not have works? Can faith save him? … You believe that there is one God. You do well. Even the demons believe—and tremble! … Was not Abraham our father justified by works when he offered Isaac his son on the altar? Do you see that faith was working together with his works, and by works faith was made perfect? … You see then that a man is justified by works, and not by faith only … For as the body without the spirit is dead, so faith without works is dead also.” (James 2:14-26)
Is it any wonder that Martin “faith alone” Luther wanted to remove the books of Revelation and James from the canon?
The Catholic Church informed believers what constitutes the full canon of Scripture, which happens to include the OT. It that sense, the Catholic Church “gave” the OT to Christians. How would the Gentile Christians have become of the OT if the Catholic Church had not approved it and not included it in the Bible? Maybe a visiting rabbi taught them about it, but somehow I doubt it.
You really ought to study the early Church Fathers - the term “Catholic Church” has been used since at least AD107:
“Around the year A.D. 107, a bishop, St. Ignatius of Antioch in the Near East, was arrested, brought to Rome by armed guards and eventually martyred there in the arena. In a farewell letter which this early bishop and martyr wrote to his fellow Christians in Smyrna (today Izmir in modern Turkey), he made the first written mention in history of "the Catholic Church." He wrote, "Where the bishop is present, there is the Catholic Church" (To the Smyrnaeans 8:2). Thus, the second century of Christianity had scarcely begun when the name of the Catholic Church was already in use.”
How Did the Catholic Church Get Her Name?