Because all the NT documents were written in one way or another for Christians
(or at least potential ones - we don't really know about Theophilus). But it's clear
from Acts & Paul's letters that the resurrection was the basis of the apostolic
missionary activity. & for people today any access we have to the apostolic message is
ultimately the NT witness.
George L. Murphy
gmurphy@raex.com
http://web.raex.com/~gmurphy/