The OP appears to ask about two things. One is biblical manuscript veracity and about historical sources regarding the existence of Christ. This answer is about the latter.
There are a number of sources that attest to Christ Jesus as an historical figure. The Old Testament, of course, prophesies of a Messiah, but who? In the New Testament, we know of Him as Jesus Christ. Are there non Scriptural sources that know of Jesus? We will look at four (Clement of Rome about 95 CE, Tacitus about 116 CE, Josephus about 95 CE, Pliny the Younger about 112 CE) who answer in the affirmative.
To be sure, these references are not without controversy, but they are satisfactorily addressed and clarified. Thus, there is no doubt that a man called Jesus Christ walked the earth and was witnessed about.
CLEMENT OF ROME
While Clement is a Christian, he is also a very early non-canonical witness to Christ Jesus.
The Church of God which sojourns at Rome, to the Church of God
sojourning at Corinth, to them that are called and sanctified by the
will of God, through our Lord Jesus Christ: Grace unto you, and peace,
from Almighty God through Jesus Christ, be multiplied.
http://www.ccel.org/ccel/schaff/anf01.ii.ii.i.html
TERTULLIAN
With Tertullian writing about 210 CE, we will back into the earlier witness of Tacitus who also wrote about Jesus Christ.
Chapter XXI:
But having asserted that our religion is supported by the writings of
the Jews, the oldest which exist, though it is generally known, and we
fully admit that it dates from a comparatively recent period—no
further back indeed than the reign of Tiberius—
http://www.ccel.org/ccel/schaff/anf03.iv.iii.xxi.html
Do we accept this account of Tiberias from Tacitus?
Reign of Tiberius 9/18/14-3/16/37 AD The Senate convened on 18
September, to validate Tiberius's position as Princeps and, as it had
done with Augustus before, extend the powers of the position to
him.[40] These proceedings are fully accounted by Tacitus.[41]
https://en.wikipedia.org/wiki/Tiberius
http://www.perseus.tufts.edu/hopper/text?doc=Perseus%3Atext%3A1999.02.0078%3Abook%3D1%3Achapter%3D10
As such, we also have Tacitus’ accounts of Christ.
TACITUS
Tacitus wrote the Annals about 116 CE. From this, we find the reference to Christians (or Chrestians) and Christ:
Consequently, to get rid of the report, Nero fastened the guilt and
inflicted the most exquisite tortures on a class hated for their
abominations, called Christians by the populace. Christus, from whom
the name had its origin, suffered the extreme penalty during the reign
of Tiberius at the hands of one of our procurators, Pontius Pilatus,
and a most mischievous superstition, thus checked for the moment,
again broke out not only in Judæa, the first source of the evil, but
even in Rome, where all things hideous and shameful from every part of
the world find their centre and become popular. Accordingly, an arrest
was first made of all who pleaded guilty; then, upon their
information, an immense multitude was convicted, not so much of the
crime of firing the city, as of hatred against mankind.
http://www.perseus.tufts.edu/hopper/text?doc=Perseus%3Atext%3A1999.02.0078%3Abook%3D15%3Achapter%3D44
JOSEPHUS
Josephus also wrote about 95 CE about Christ, His brother James, and Christians in Antiquities 20.9.1 and 18.3.3:
Festus was now dead, and Albinus was but upon the road; so he
assembled the sanhedrim of judges, and brought before them the brother
of Jesus, who was called Christ, Antiquities, 20.9.1
http://www.ccel.org/ccel/josephus/complete.ii.xxi.ix.html
Now there was about this time Jesus, a wise man, if it be lawful to
call him a man; for he was a doer of wonderful works, a teacher of
such men as receive the truth with pleasure. He drew over to him both
many of the Jews and many of the Gentiles. He was [the] Christ. And
when Pilate, at the suggestion of the principal men amongst us, had
condemned him to the cross,541 those that loved him at the first did
not forsake him; for he appeared to them alive again the third day;542
as the divine prophets had foretold these and ten thousand other
wonderful things concerning him. And the tribe of Christians, so named
from him, are not extinct at this day.
http://www.ccel.org/ccel/josephus/complete.ii.xix.iii.html
PLINY THE YOUNGER
He writes to the Emperor Trajan about 112 CE regarding Christ and Christians:
They asserted, however, that the sum and substance of their fault or
error had been that they were accustomed to meet on a fixed day before
dawn and sing responsively a hymn to Christ as to a god, and to bind
themselves by oath, not to some crime, but not to commit fraud, theft,
or adultery, not falsify their trust, nor to refuse to return a trust
when called upon to do so.
http://www.earlychristianwritings.com/text/pliny.html
CONCLUSION
In sum, it is only the uninformed and misguided who do not believe there was a man called Jesus Christ who walked the earth and had followers after his death (and resurrection) that were called Christians. Historic references to Christ may be found from foe and friend. The real dispute is not whether Jesus Christ lived or even if He was crucified or had followers, but whether He rose from the dead.
For further references
https://probe.org/ancient-evidence-for-jesus-from-non-christian-sources-2/#text10
http://www.josephus.org/testimonium.htm
https://en.wikipedia.org/wiki/Josephus_on_Jesus
http://earlychristianwritings.com/testimonium.html
https://en.wikipedia.org/wiki/Tacitus_on_Christ