A false prophet's doctrines and their fruit are not 2 different things as you assume. People practice what they believe, so the person's doctrines are indicated by their fruit. If a person's fruit is bad, it's because their doctrines are bad. That's why Isa. 8:20 says someone is a false prophet if their doctrines aren't aligned with the Law.
Quoting scripture doesn't mean anything. Satan can quote scripture too. He did it when he tempted Christ, and he still does it today through false prophets.
That is usually the go-to passage for trinitarians, but everything from "in heaven" to "on earth" was inserted by trinitarian translators. It is not in the inspired text, not to mention the fact that this very intentional addition disrupts the entire message John taught throughout his first epistle.
Ecclesiastes says it is the duty of ALL human beings to keep Gods commandments:
"Now all has been heard; here is the conclusion of the matter: Fear God and keep his commandments, for this is the duty of all mankind." - Ecc. 12:13
There isn't a single passage where it says it was only the duty of Jews to keep God's commandments, especially because Jews didn't exist as a people until the nation of Israel split into 2 separate kingdoms about 500 years after Moses died. There is also the matter that God says Abraham kept His commandments in Gen. 26:5, and Abraham was not a Jew. As Paul taught in Romans, sin could not exist where God's commandments didn't exist. Seeing as though the Bible says Adam sinned, it obviously means he violated commandments that were already enforced.
A person doesn't have to make outright claims of being a prophet in order for the label to be applied to them. The Bible interchangeably refers to false prophets as being the same as false teachers because both presume to speak on God's behalf and people are led towards the worship of a false god in the process. 2 Pet. 2:1, 1 Jhn 4:1, Matt. 24:11, and Eze. 22:25-28 are just a few examples where this is clearly taught. According to what the Bible says, liars get their guidance from the spirit of error, not the Holy Spirit.
"For by Him(Christ) all things were created that are in heaven and that are on earth, visible and invisible, whether thrones or dominions or principalities or powers. All things were created through Him and for Him." - Col. 1:16
"and to make all see what is the fellowship of the mystery, which from the beginning of the ages has been hidden in God who created all things through Jesus Christ;" - Eph. 3:9
Jhn 1:1 clearly describes 2 God Beings who have always existed alongside each other as separate Beings.
No, people have a problem with the orthodox doctrines, traditions, and writings of the "church fathers" because those things clearly contradict the inspired word of God. The "church fathers" were not apostles, nor were they ever given an apostle's authority. And as such, there is no biblical reason to view their writings as having the same amount of authority or more authority than the apostles' writings.