Just a point of clarification: Protestants accept creeds and confessions where they help clarify doctrinal truths described in scripture. Thus we accept things such as the Nicene Creed, the Apostle Creed, or the Westminster Confession of Faith (London Baptist Confession if your a Baptist). Normally these creeds and confessions are rock solid doctrinal understanding passed down through the ages to fight against heretical doctrine. Sadly, few Protestants understand the remarkable work put together by our forefathers in understanding scriptural truths through the creeds and confessions. A good confession will have its basis in clear understanding of scriptural truths.
What Protestants reject are new truths which are contrary to sound teaching already established. Thus if the scriptures state, “All have sinned...”, then we cannot teach that certain people were sinless. Not only does it go against the creeds and confessions established, but it also goes against scripture. A well founded creed will be based on rock solid scripture.
He was obviously talking about the traditions/teachings that they had learned from him. Paul then ensured ALL believers were knowledgeable of those teachings by writing them down. Copies were made and distributed so that eventually all the local assemblies possessed the writings. He also admonished believers, "Do not go beyond what is written." (I Cor. 4:6) Take all that (plus more) and put it together and it sure sounds to me like we should always measure what we teach and believe by what God's word teaches and not go beyond it and end up like the Pharisees of whom Jesus told "thus you nullify the word of God by your tradition that you have handed down". (Mark 7:13)
Actually, what CHRISTIANS reject is new claims of truth or claims of NEW truth, that are contrary to both revealed Scripture and sound teaching based on Scripture that is already established.