Mathew, Mark and Luke have noticeable similarities while John is quite different.  The first three Gospels agree substantially in language, content and in the recorded order of events.  Consequently, these three books are called the Synoptic Gospels.  syn means together; optic means seeing.  A comparison shows that 91% of Mark’s Gospel is contained in Matthew while 53% of Mark is found in Luke.  Such agreement raises questions about the origin of the Synoptic Gospels.  Several explanations have been proposed.
Tradition had become so stereotyped that it provided a common source from which all the Gospel writers drew.
Synoptic authors all had access to an earlier Gospel, now lost.
Written fragments had been composed concerning various events from the life of Christ and these were used by the Synoptic authors.
Synoptic writers drew from each other with the result that what they wrote was often similar.
The Gospel of Mark and a hypothetical, lost document, called Quelle or Q (meaning source), were used by Matthew and Luke.
Mark and Luke drew from Matthew as their main source.
A combination of the above.
Regardless of which explanation is correct, Christians believe that God inspired certain writers to preserve His infallible and inerrant thoughts as the long lost, but unique, autographs we now refer to as Matthew, Mark and Luke.  Each subsequent manuscript and manuscript fragment is a pure representation of one of these unique autographs.
The entire New Testament has not only survived in more manuscripts than any other ancient writing but it has survived in a much purer form than any other great book.  Not one shred of Christian doctrine hangs on debatable text.