What is it & why do we need to understand it
Modern Criticism of 19th century basically centered in Germany the Gospel has, in my understanding, perpetuated the modern critical interpretation by the liberalism thinkers of today. Some of the tangible effects today are like liberal Christians love the kindness and compassion of Christ but reject his resurrection and virgin birth and even the historicity of Christ!
Most average church goer Christian would probably respond so what? It hardly affects my life. But the reality is, it has affected millions of people especially the millennial generation with this being taught in colleges, being the liberalism colored lens. If we want to reach out to the educated mass today, we must understand better the liberalism Modern Criticism of the Gospel.
This is very problematic because essentially, they the liberals refuse to back down from being called a Christian, while rejecting the miraculous and the virgin birth f Christ and the resurrection of Christ, because they are amazed with the person Jesus Christ, and loving his virtuous deed and teachings, like the sermon on the Mount, the good Samaritan, stop being judgmental, feeding the poor, kind and compassionate, indeed Jesus is a par excellence moral character to them, but alas, nothing divine. And why? Because in their mind, this doesn’t fit into or congenial to their scientific minds and worldview! So they actually dissected the gospel and divided as they wish to try to “fit” into the science driven worldview.
A famous liberalism thinker scholar in Germany called Rudolf Bultmann in the 19th century has impacted much modern liberalism. Robert Strimple wrote: While he actually summarized the theology of the New Testament that is set in that mythical cosmological framework:
The aeon is held in bondage by Satan, sin, and death and hastens to its end. That end will come very soon, and will take…a cosmic catastrophe. The Judge will come from heaven, the dead will rise…”In the fullness of time, God sent forth his Son, a pre-existent divine Being, who appears on earth as a man. He dies the death of a sinner on the cross and makes atonement for the sins of men…. The risen Christ is exalted to the right hand of God in heaven and made “Lord” and “King”. He will come again on the clouds of heaven to complete the work of redemption, and the resurrection and judgment of men will follow.”
This all fit into the biblical gospel very well, and we rejoice. But no! But Bultmannn insisted, it is impossible for any modern person to receive in such a message: “Man’s knowledge and master of their world have advanced to such an extent through science and technology that it is no longer possible for anyone seriously to hold the New testament view of the world-in fact there is no one who does. 
What catches my attention is this line: “The mythical eschatology is untenable for the simple reason that the Parousia of Christ never took place as the New Testament expected. History did not come to an end,…it will continue to run its course.” This is a wrong exegesis statement from Bultmann and it has such huge significance, because that is the breaking point for him to turn away from true Christianity sadly. For a brilliant mind to land in such dark philosophical thinking, is very grieving as he and a lot of other liberalism scholars impacted the books and syllabus we read in colleges today and for the past centuries. Why blame it on the return of Christ (parousia) not happened yet? And that caused Bultmannn to conclude the mystical eschatology as untenable?? That means the supernatural miraculous, and the final judgment. So sad. Maybe he should have exegete the timing of the parousia, like a day is a thousand years to the Lord, and a 1000 years like a day.
He tried so hard to reconcile the mythical side of the bible to the reality of science of his days. He even tried to call to delete the mythological (supernatural for us) in the New Testament as “husk”, so that the “kernel” of truth might remain (as Harnack had suggested)? But he said that one couldn’t delete the husk because the truth (kernel) is actually embedded in the so-called husk. Therefor his conclusion is not to delete the mythological elements of the NT, but to transform that kerygma into the conceptual framework with which modern people operate. You see the attempts of these philosophers go deep. In my view these folks have just put science onto a pedestal that’s idolatry. Ironically, science can’t explain conclusively the black hole and tons of stuff including the evolution in our 21st century, let alone their 19th century.
So Bultmannn went ahead and reconstructed the whole Gospels, calling it to decontextlaization and recontextualization. Bultmannn teaches that we must understand the Gospel mythology, not cosmologically but anthropologically, or more specifically, existentially.
The biggest problem is the definition of human nature, Being. Their problem can be summarized in Boltzmann’s question: “Must we understand it as the cross of Jesus as a figure of past history? Must we go back to the Jesus of history?”
Strimple writes: Christ’s death and resurrection saved all who believed in him because Jesus is God’s Son, whom he appointed as heir of all things, and through whom God made the universe. The Son is the radiance of God’s glory and the exact representation of his being, sustaining all things by his powerful word” (Heb 1:2-3) This is the one whom the angels worship (1:6) and who is addressed as “God” (1:8), but “who was made a little lower than the angels…so that by the grace of God he might taste death for everyone, “ and who “ is not ashamed to call” those whom he makes holy “brothers” and who “shared in their humanity”. (2:9,11,14) In other words, the significance of his work is grounded in the significance of this person as the God-man. 
The question that confronts us today is that whether we are
willing to understand ourselves as one crucified and risen with Christ.
 Strimple, Robert The Modern Search for the Real Jesus P&R Publishing 1995, New Jersey p120