The Bible does warn against false teachers, prophets and other gospels. It says that the scripture is good for many things. Prove all things, hold that which is true. What other standard is there but scripture to know these things about God?
In the Bible, the apostles wrote, stick to our letters and what we said in public in from of many reliable witnesses. If you have some teaching outside of the Bible, I'll need to know the reliable witnesses and the chain of custody. Otherwise, we're talking hearsay.