It's often been said you cannot use the bible to prove the bible, this is presupposing that the bible is one single book and that other books should be consistent. Textbooks are collections of the best works available, compiled for the purposes of teaching truth. If one of the best truthful writings was left out of a textbook that would be negligent. In the same way the bible is a collection of books of which were the most accurate, and consistent works of the time. Would it be sufficient to use the book of Luke or Acts to prove the bible if they were not in the bible today and it was consistent with the other gospels? However to support the critics we do have over 10 external non-Christian sources that verify different parts of the Bible. Sources such as:
Josephus, a Jewish historian.
Thallus (52 AD),
Tacticus(56-120AD),
Mara Bar-Serapion (70AD),
Phlegon (80-140AD),
Pliny the Younger (61-113AD),
Suetonius (69-140AD),
Lucian of Samosata: (115-200 A.D.),
Celsus (175AD),
Jewish Talmud (400-700AD),
The Toledot Yeshu (1000AD).
Source: http://coldcasechristianity.com/2014/is-there-any-evidence-for-jesus-outside-the-bible/