The church has made many attempts to rid itself of allegorical interpretation for a very good reason. It is based on Greek rhetorical invention, which has no means of validating, or preventing a free-for-all. The reason it has been unsuccessful is because there are so many hints that there is a deeper meaning or a parallel to the life of Christ. Why do so many second sons preempt the first born? for instance. What's with all the 3's, 40's and 7's? Add to this God's own tease:
It is the glory of God to conceal a thing: but the honour of kings is to search out a matter. —Proverbs 25:2
The rules which separate Greek allegory from sensus plenior (which cannot be denied to be a subclass of allegory) are these:
These rules are discerned from the Scriptures using the same hermeneutic as sensus plenior:
Since God has said that not a jot or tittle will pass away, until one knows why each jot and tittle is there, a complete understanding has not been derived. (This keeps us humble)
Since man shall live “..by every word”, a doctrine is not sound until it sums up and includes all that God has said about it. (This keeps us searching)
Since every word must be established by two or three witnesses (Mt 18.16, De 19.15), every shadow must have at least two supporting scripture witnesses. (This keeps us rigorous in methodology)
Since God’s word is established forever (Pr 12.19,Ps 119.89), a shadow means the same thing everywhere is it used. So, since a donkey is a shadow of a prophet, everywhere there is a donkey, it is a shadow of a prophet. This rule alone makes the shadows humanly impossible to fabricate. (This keeps us in awe)
The riddle of Samson tells us Christ is the answer to all the riddles (1). If the shadow doesn’t look like Christ, it isn’t a good shadow. (This keeps us focused)
And since we are to “let everyman be a liar and God be true” (Ro 3.4), outside references are not required to solve the riddles and see the shadows. (This keeps us devoted)
A properly discerned allegory should have the same authority as literal interpretations. We should not trust such authority to anything that is not verifiable.
Scholtz has done a good job of documenting the beginning of the slippery slope in the interpretation of Parables by over 20 popular modern scholars. Many of these men claim to be expositors.
"Collective meaning and specific, prophetic reference in the parables of Matthew 13" by Jacob Jan Scholtz, Stellenbosch University http://scholar.sun.ac.za/handle/10019.1/85594
If we summarize the modern teaching of the parable of the mustard seed, you learn that the kingdom starts off small and grows and grows and if you have faith like that, you can move a mountain to the sea, which no one seems to be able to do. The birds may be angels, demons or gentile nations. The tree is likely a great empire, or ecclesiatical-political system, or world power. And most question or try to explain why Jesus said the seed was the smallest, when we all know that it isn't.
This is a pretty good documentation of free-for-all interpretation.
Now use the exegetical method of sensus plenior which recognizes allegory everywhere, but derives it's meaning from the formation of words and their meanings, and correlation with other scripture.
Jesus was the seed of the woman in Genesis 3 who had the bruised heel. He was the least of the seed because he served us all on the cross. 'Mustard' in Greek' sounds like 'bruised by anger' in Hebrew.
Free-for-all allegory suggests that 'leaven', in other parables, represents sin. But The kingdom of heaven is like leaven, so it is a bit difficult to explain when the rules above are applied. Why would the Hebrew rid themselves of leaven/sin before Passover/cross when we do not get rid of our sin before the cross? Leaven represents teaching.
Back to the parable of the mustard seed. The kingdom of heaven is like leaven/teaching of the cross. The parable points back to the prophesy of the seed of the woman. The rest of the parable can be exegeted in the same fashion. Jesus became the great teacher of the cross (herb) he then fulfilled it on the cross (tree) and those who live in the spirit rest in the cross. These are all discerned by finding the OT source for the NT teaching.
To understand the teaching of moving the mountain to the sea, you must parse out the meaning of 'water' מים by the letter to find the Father and Son. Then cross reference to Jesus's teaching at Sychar to the woman at the well. Though she worshiped on the mountain, she would worship in Spirit and Truth. The Father is Spirit; the Son is Truth, so she would move the mountain, the place of her worship, to the water, the Father and the Son.
Proper allegorical interpretation derives from the meaning of the words and by correlation with other scripture. It is validated by the same teaching in other places.
*formation of words is called: Notarikon - Interpretation by dividing a word into two or more parts in the 32 rules of Rabbi Eliezer ben Jose de Galili
(1) "Plowed with my heifer" Plowed - turn over the earth(ly) (or literal). Heifer- the sacrifice (Christ). Use what you know of the 'mystery revealed' (Eph 1.9, 3.3) to find what was concealed previously (Pr 25.2)