Mathew 7:15-20 An Ironic Warning

Jesus = good/bad tree cut down and cast into the fire

Summary:  Every element in this passage links it to the Genesis account of the fall.  Jesus' ironic warning to "beware" shows us the impossibility and desperation of discerning good and evil in a cursed world.  We are now after 4000 years, in much worse condition and the deceiver has a much better disguise.  There are no good trees but one; the only good tree is the one who became bad for our sake and was cut down and thrown into the fire.  We, the bad trees, are the recipients of that grace; we are his good fruit.  Bad trees become good fruit!

By taking us back into the Genesis account of the Fall, Jesus reminds us that every thing in their history, in Jesus' life and action and, indeed, everything in the universe is part of one over-arching story of the nature and work of Christ.  No passage in the bible has been fully explained until it is placed within that larger narrative.   It is as if the Second Person of the Trinity is picking up and continuing the same conversation with Adam and Eve, now in the person of their cursed progeny 4000 years later.
"Beware"
How do you do that?  You use the faculty gained from eating the fruit of the tree of the knowledge of good and evil. With stinging irony the same incarnation of God that just cursed them and the ground they live, work and die on says to Adam and Eve's cursed progeny, "Well, now that you have become like one of us, knowing good from evil, how are you doing?  Think you can tell the difference between  true and false spokesmen for me?  Okay, let's see."
(Another guiding principle of Christ-centered preaching is that the ultimate purpose behind every law, rule, exhortation, warning from the mouth of God, when read or heard deeply, convinces us of our total inability ever to even begin to perform  up to God's standards -even when empowered by God's Spirit -  and drives us to look for a Savior.)
"ravenous wolfs"
Prowling about seeking whom he may devour.  Again the same confrontation between man and deceiver.  It didn't go well the first time; how well can it be expected to turn out this time?  If not Satan himself, this cetainly is one of his subtly deceptive employees. 
"Sheep's clothing"
21 And the Lord God made for Adam and for his wife garments of skins and clothed them. 
The wolf comes preaching the gospel!  "Look, I am one of you!  I have been covered by the blood of the Slain Lamb.  Even in the midst of judgment God has shown me his love and mercy.  I am a Christian and I wear the same gospel covering as Adam and Eve."
(He just doesn't tell you how he received this covering: he went out and killed it himself.  He worked for it.)
Doubly Deceived
Notice how much easier it will be this time for the wolf/deceiver to fool Jesus' audience! The great great grandchildren are now suffering these thousands of years out of fellowship with God and surrounded by a cursed, thorn and thistle filled environment.  In fact the curse flows from man to earth!  Every time a man takes a step - every time one dies and is returned to the soil, corruption radiates out from the point of contact.  Their corruption of their senses and intellect and character have been compounded for generation after generation.  Sweating out a brief existence in an angry world, where is a sweet trusting confindence in the love and care of an all knowing, loving Father going to be learned?  And failing to discern the most basic fact of the universe, how likely are we to get anything else right?  These depraved products of 1000s of years under the curse of God are going to know a good preacher from a bad one?  Ha!
And to make it even easier for the Deceiver, he now has the perfect disguise.  He comes as a Christian preaching the gospel! 
"But wait!", you say, "Satan would never preach the gospel!"
When Satan heard you say that he said to himself, "That's it!  The perfect disguise, the one they could never believe I would wear!  I won't attack God, I'll come preaching his sovereignty, his righteousness, his holiness, his justice.  It's perfect!"
What did you think a wolf in sheep's clothing was going to preach?  That Jesus isn't the Messiah?  That God doesn't exist?  That he didn't die for our sins?  That there is no hell and no devil?
We're infinitely more corrupted than Adam and Eve and Satan has the perfect disquise.

thorns and thistles
cursed is the ground because of you; in pain you shall eat of it all the days of your life;18 thorns and thistles it shall bring forth for you; and you shall eat the plants of the field.
19 By the sweat of your face you shall eat bread, till you return to the ground,for out of it you were taken; for you are dust, and to dust you shall return.”

Are grapes gathered from thornbushes, or figs from thistles?

God (in the person of Jesus?) has just sent them out of the garden to continue pursue their vocation as gardeners, but now as alien laborers and gleaners. The same Jesus now shows up outside the garden and asks Adam and Eve's children this rhetorical question loaded with bitting irony. 

healthy trees and bad trees
17 So, every healthy tree bears good fruit, but the diseased tree bears bad fruit. 18 A healthy tree cannot bear bad fruit, nor can a diseased tree bear good fruit.
Driving the spike of irony deeper still, Jesus brings these fruit pluckers face to face with the worst decision they ever made and reminds them what they have been painfully learning for all these generations.  "Your presumptious and disobedient harvest of wisdom and discernment is useless because there is only one kind of tree in the orchard: bad!"

Cut down and thrown into the fire
Here's the line that tests the ESV idea that all such judgment looks forward to the wrath poured out on Jesus.  If your new birth is most essentially the ability to see the beauty of Christ in the cost He paid for your redemption, quiet your intellect for a moment that your heart can be heard.  What does it do for you to know that the One Good Tree was cut down and cast into the fire that He might bear fruit precious enough and good enough (to Him)  to die for, namely us,  His cursed and diseased orchard made whole.
20 Thus you will recognize them by their fruits
Up until now I have been arguing that this passage must not be read merely as Christ showing helpful concern and warning to "beware"  as if we could, but rather as full of irony and pointing out how helpless we are under His curse.  That point having been made, this passage does, after all, help you recognize a wolf in sheep's clothing: they will preach right over verse 19 as if "the gospel" were at least in part a terrifying restatement of what happens to us when we don't pass inspection.  They will tell you that the Gospel is about what you need to do and what will happen to you if you don't.  This is good news?  No, the Gospel is not about what you need to do but about what another has done for you.  Until your heart has been warmed by the sight of the One Good Tree being cut down and thrown into the fire for you, you can't do your first good deed.  The only good trees are those who although they have never produced a single good fruit know themselves to be infinitely loved while bad!.  "The only proper motivation for a life of holiness is the knowlege of sins forgiven."