That’s where cultists start from: the Trinity and the canon of the NT were “just made up”. Do you believe Paul when he commanded the Thessalonians to hold fast to the traditions they had received from him, whether by word of mouth or by letter? (2 Thess 2:15) Or was he just making that up?
Any tradition received directly from an Apostle would be oral scripture. The Apostles were prophets. Thus, their pronouncements, oral or written, is scripture - special revelation. Since they are long dead all we have left is written scripture. This is much different from a denomination just making up something up with no Apostolic foundation.