Mark 4:2 And he taught them many things by parables, and said unto them in HIS DOCTRINE,
3 Hearken; Behold, there went out a sower to sow:
2 John 9 Whosoever transgresseth, and abideth not in the doctrine of Christ, hath not God. He that abideth in the doctrine of Christ, he hath both the Father and the Son.
God's people often find truth in parables which results in the formation of doctrine. The End of the Church Age doctrine is an example of this. It was mainly arrived at through the understanding of historical parables. Which is why God's elect point to the historical parables found in the book of Jeremiah, for instance, and say, "Judah and Jerusalem are types and figures of the New Testament churches. As God brought judgment on unfaithful Judah He has done likewise by bringing judgment on the unfaithful churches and congregations of the world."
Some protest and say you can't form a doctrine based on the understanding gleaned from a parable. And yet, Mark 4:2 declares that Christ spoke in parables and when He did so that He taught them in "His doctrine".
Therefore, things learned concerning the Bible's mysteries (parables) as God reveals them to us by opening up our eyes to understand them-is indeed a true and proper basis for forming legitimate doctrine.