It’s not too complex actually. Take a look at this (it’s from the link above so I suggest you visit that site):
Older strategies that support the historical reliability of the New Testament often begin their case by pointing out that the New Testament documents enjoy superior manuscript evidence.* Recent indications are that the New Testament is supported by more than 5500 copies and partial copies in Greek and other languages, while most ancient classical Greek and Roman texts have fewer than 10 each.* Moreover, there is comparatively little significant variation between these manuscripts, even when they are derived from different textual families.
While this extraordinary quantity and quality of the available texts does not tell us if the New Testament writings are historically reliable, most scholars think that the far more manuscripts and portions do indicate that we essentially have what the authors originally wrote.* This is obviously a crucial point to begin.
Further, the New Testament copies are much earlier—that is, closer to the original writings—than the classical texts.* Most of the New Testament is available from copies that are only 100-150 years after its completion, while a copy of the entire New Testament dates from about another 100 years after that.* In contrast, the classical counterparts generally date from 700-1400 years after their original compositions.* This enormous difference significantly closes the distance between the authors and the earliest copies, placing the dates of the New Testament copies much closer to the events themselves.* This makes it at least possible that the biblical writers were in a better position to know what actually occurred.[2] etc, etc
It even covers Jesus’ sayings.
I do believe the Gospels are reliable even if there are no originals