The phrase 'Historical Jesus' is one that came out of the enlightenment, particularly in Germany. The general idea behind it is that "Jesus" as a man who lived and was recorded was different to "Christ", or the figure underlying Christianity. To the thinkers of the enlightenment, a supernatural redeemer did not fit with the view they were coming to of the world, whereas they had a strong appreciation for an enlightened, moral teacher who had some really good ideas. This view was particularly developed by Hermann Samuel Reimarus, who denied the supernatural nature of Jesus.

Partly as a reaction to the perceived ills of Christianity at the time, especially in the minds of rationalists, it was thought that their must be an enormous gulf between the Jesus who existed and the Christ of faith. These thinkers believed that by discovering the nature of 'historical Jesus' they would be left with a more credible version of Christianity, and would be able to shed the unnecessary and (in their opinion) outdated dogmatic traditions.

These ideas were originally espoused in the 17th century, were revisited in the late 18th century when the writings of Reimarus (1694-1768) were posthumously published. Reimarus argued that the foundations both Judaism and Christianity were fraudulent and this argument was presented in his book An Apology for the Rational Worshiper of God. Throughout this work he went through the entire bible and subjected it to the rationalist criticism that had become popular for other philosophic and scientific works of the time, but he never published his work during his lifetime, and it didn't come out until 1774.

The most shocking claim to readers who saw it then denied the historicity of the resurrection, but also claimed that the gospels had been tampered with by the early Christians, and were therefore fraudulent. He then claimed that the intentions of the disciples were quite different to those of Jesus, and that the fraudsters had twisted Jesus's story to make him fit the Jewish apocalyptic idea of a Messiah.

Now how does this relate to C.S. Lewis? That's in large part due to a guy named Albert Schweitzer (1875-1965) You may notice he lived in almost the exact time as Lewis (1898-1963), and he wrote a book called The Quest for the Historical Jesus, in which he heaps praise upon Reimarus's theories. Reimarus had almost no followers when his writings were published, but Schweitzer's book was popular and kicked off a lot of debate about whether or not Jesus truly was divine or whether he just had some good ideas.

C.S. Lewis was famously a christian, but more specifically He was a Pauline Christian, and to him the deification of Christ, (God became human) was one of the most important tenants of his philosophy. To Lewis, without divinity the Jesus story has nothing in it. One of his most famous quotes from Mere Christianity:

I am trying here to prevent anyone saying the really foolish thing that people often say about Him: 'I'm ready to accept Jesus as a great moral teacher, but I don't accept his claim to be God.' That is the one thing we must not say. A man who was merely a man and said the sort of things Jesus said would not be a great moral teacher. He would either be a lunatic – on the level with the man who says he is a poached egg – or else he would be the Devil of Hell. You must make your choice. Either this man was, and is, the Son of God, or else a madman or something worse. You can shut him up for a fool, you can spit at him and kill him as a demon or you can fall at his feet and call him Lord and God, but let us not come with any patronising nonsense about his being a great human teacher. He has not left that open to us. He did not intend to.

So in the Screwtape Letters he is saying that the 'tempters of Hell' have implanted the idea of a historical, non-divine Jesus in order to confuse and disrupt Christian worship.

TL;DR: Lewis believed that the divinity of Jesus was essential to Christianity, and the 'historical Jesus' was an enlightenment theory about Jesus just being a good moral teacher and not God himself. In the Screwtape Letters Lewis is claiming that the HJ theory is the work of the devil, in order to make people believe less in Jesus as God.

Sources:

Faith Seeking Understanding - Daniel L Migliore

Christian Theology - Alister E. McGrath

Mere Christianity - C.S. Lewis

The Quest for The Historical Jesus - Albert Schweitzer

Systematic Theology - Robert Duncan Culver

The FAQ here has much to say about the historicity of Jesus. You could, also, try asking more specific questions at either /r/AskBibleScholars or /r/AcademicBiblical.

The basic presupposition of critical-historical scholarship on the figure of Jesus over the last few hundred years is that the world-shattering impact of this 1st century Jew’s brief public career was soon domesticated by the early Christian communities, who found the radical challenge of his life and teachings almost impossible to live with. The central task of historical reconstruction, then, is to dig beneath the mythic and supernatural encrustations that were built up around Jesus and then ossified in the developing streams of early the Church (e.g. the Virgin Birth, the ‘only begotten’ son of God, substitutionary atonement, the pre-existing Logos, the Second Coming, etc.) in order to re-activate the "historical Jesus" - the earliest memory of this Galilean sage prior to his subsequent inscription within the New Testament canon. Now, in contrast to much modern day skepticism about the historical reliability of the Christian gospels, the initial case to be made here is that the earliest memory of Jesus has indeed survived the historical process of oral and written transmission in the 1st century with his characteristic oral teachings still intact. Or more specifically, the original signature of this itinerant Nazarene Jew’s historical voice-print is embedded within narrative core of his most memorable short stories and parables.

And so what would the parables of Jesus look like if we extracted from them all subsequent Christian interpretation and theological embellishment over the past two millennia, and let the event that simmers away within his proclamation of the Kingdom be unleashed in its first, disturbing, immediacy? Here’s the briefest possible summary:

Good Samaritan Luke 10:25-37: the agents of neighbourly love, i.e. the devout Pharisee & Levite priest, are exposed as morally and religiously bankrupt; the morally and religiously bankrupt, i.e. the half-caste Samaritan, is an agent of neighbour love.

Prodigal Son Luke 15:11-32: the younger wayward son, who is lost in rebellious exile from home, is saved with the father’s homecoming feast, the older dutiful son, who is saved in righteous obedience to his father’s home, is lost in rebellious exile from the homecoming feast.

Vineyard Workers Matthew 20:1-16: the privileged status of those who started work at day break is the source of their envy and their sense of injustice; the envy and injustice of those who started work just before sundown, is the occasion for their privileged status, (when both groups receive equal pay from the vineyard owner).

Pharisee and Tax Collector Luke 18:10-14: the self-righteous Pharisee, who is saved in as a Temple insider - is lost, just as the sinful Tax Collector, who is lost as a Temple outsider - is saved.

Two Sons Matthew 21:28-32: the acceptance of the first son is a refusal to work in the field, and the refusal of the second son is an acceptance to go work in the field.

Friend at Midnight Luke 11:5-8, where: the refusal of a friend becomes the acceptance of stranger, while the persistence of a stranger is the faithfulness of a friend.

Shrewd Manager Luke 16:1-8: where: a shrewd business manager is condemned his employer as a fraud, as this fraud is praised by his employer as a shrewd business manager.

Unjust Judge Luke 18: 2-8: we see: the refusal to back down of a fearless judge is a merciful delivery of justice, just as the merciful pleas of the widow for justice are a fearless refusal to back down.

Unforgiving Slave (Matthew 18:23-35): an occasion for judgment is act of forgiveness, an occasion for forgiveness is an act of judgment.

Rich Fool (Luke 12:16-21, Gospel of Thomas 63): the wisdom of working to secure ones future is foolish hoarding, the foolishness of an unsettled and uncertain future, is wisdom and security.

Final Judgment (Matthew 25:31-46): those who know God’s name are damned in never serving the poor, while those who serve the poor are saved even though they never knew God by name.

Great Banquet (Luke 14: 16-24, Matthew 22:2-14, Gospel Thomas 64): the ones who to get in are the ones who are expected to be left out; while the ones who are left out are the ones who are expected to get in.

Rich Man and Lazarus (Luke 16:19-31): the rich man who is glorified in this life is destitute and without hope in the afterlife, just as Lazarus who is destitute and without hope in this life is glorified in the afterlife.

Wedding Feast (Matthew 22: 1-14): the outsiders are in, just as the insiders are out.

Treasure in the Field (Matthew 13:44) finding is losing and losing is finding.

Pear of Great Price (Matthew 13:45-46): looking through many is finding one, selling many is buying one.

Father’s Good Gifts (Matt 7:9-11, Luke 11:11-13): the suggestion of false gifts is an affirmation of good gifts, while the affirmation of good gifts is the suggestion false gifts (compared to the good gifts of the Father)

Talents (Matthew 25:14-30): the risk of losing what has been given is to save one’s life, to save what has been given is to risk of losing one’s life.

Slave at Duty (Luke 17:7-10): an occasion for grace is a demand for good works, while the occasion of good works is a confession of grace.

Faithful and Wise Slave (Matthew 24:45-51, Luke 12:42-46): to be a slave in the master's absence is to possess all the riches in his presence, while to possess of all the master's riches in his absence, is to be a slave in his presence.

By unsettling the binary oppositions that constitute the very form and content of human language, Jesus' discourse of radical paradox is a skilful means for a piercing through our protective defences and exposing the defects in any system that justifies the status quo by interrupting our pre-given horizons of meaning with series of subversive reversals in the expected story-line at its deepest level that put language under pressure and puts into question our taken for granted assumptions about reality.

And while the world-shattering impact of this 1st century itinerant Jew’s paradoxical sayings were soon domesticated by the early Christian communities, if we dig beneath the mythic and supernatural encrustations that were built up around Jesus in the developing streams of early the Church (e.g. the Virgin Birth, the ‘only begotten’ son of God, substitutionary atonement, the pre-existing Logos, the Second Coming, etc.) and re-activate the mind-bending event that these traditions harbor prior to his subsequent inscription within the New Testament canon, then the paradoxical secret at the heart of Jesus' most memorable teachings not only demonstrates the historical existence of this itinerant Nazarene rabbi in 1st century Palestine, but also proves to us that the earliest memory of this Jesus has indeed survived the historical process of oral and written transmission with the original signature of his historical voice-print still intact.