The early Church fathers taught that the New Testament was the final authority for Christians. Anyone requiring you to believe a doctrine that is not clearly taught in scripture is to be ignored. there  never was any secret doctrine handed down by the apostles that is not clearly taught in the scriptures.

Here is what the early church fathers said:

• Irenaeus 178 AD
• Against Heresies 3.2 - Heretics allege truth was delivered not by written documents, but by oral tradition. Without this oral tradition you can't properly interpret the Scriptures.
• Against Heresies 3.3 - The Apostles did not pass down any hidden wisdom, just the scriptures. If an older written document contradicts the newer written document we believe the older. 
• Against Heresies 4.26 - Obey the presbyters who are in the Church,--those who possess the succession from the apostles; together with the succession of the episcopate, who have received the certain gift of truth. But it is also incumbent to hold in suspicion others who depart from the primitive succession. Avoid all who do not hold to the doctrine of the Apostles, including Presbyters.
• Against Heresies 4.33 - The doctrine of the apostles, has been guarded and preserved without any forging of Scriptures, as a very complete system of doctrine. Neither receive addition to or suffer curtailment from its truths. Read the word of God without falsification, lawfully and diligently explaining them in harmony with the rest of the Scriptures. 
   
• Clement of Alexandria 192-202 AD
• Stromata book 2.4 - There never was any secret doctrine handed down by the apostles, just the scriptures. Only the heretics say there is secret doctrine from the apostles which you must know to correctly understand the scripture.
• Stromata 2.17 - To be true, a thing must be proven scientifically, by comparison and testing. 
• Stromata 7.15 - True Christians do not divide the body of Christ, but heretics try to.
• Stromata 7.16 - Do not accept any teaching that is not clearly taught in the scriptures. It must be proven logically and completely from the scriptures or it is just an opinion. Any one who divides the body of Christ with opinions is sinning against the body of Christ.
   
• Tertullian 190-210 AD
• Prescription against heretics 1.25 - The apostles did not keep any secret doctrine, but taught it all openly. Only heretics teach a secret gospel or letter or teaching.
• Prescription against heretics 1.26 - The apostles did not give special information to favorite friends.