Scripture is the written words of God through prophets, apostles, and evangelists. So divine supernatural inspiration makes certain books Scripture and others non-inspired.
Regarding the last twelve verses of Mark, Codex Sinaiticus shows A BLANK SPACE within the Gospel of Mark which is exactly equivalent to the space required for the last twelve verses of Mark. That in itself is proof that those verses were removed from that document. But the majority of manuscripts do have this passage.
Here is a summation of the book by John William Burgon whose full title is:
The Last Twelve Verses of the Gospel According to S. Mark: Vindicated Against Recent Critical Objectors and Established.
Dedication.
Preface.
Chapter I. The Case of the Last Twelve Verses of S. Mark's Gospel, Stated.
Chapter II. The Hostile Verdict of Biblical Critics Shewn to Be Quite of Recent Date.
Chapter III. The Early Fathers Appealed to, and Observed to Bear Favourable Witness.
Chapter IV. The Early Versions Examined, and Found to Yield Unfaltering Testimony to the Genuineness of These Verses.
Chapter V. The Alleged Hostile Witness of Certain of the Early Fathers Proved to Be an Imagination of the Critics.
Chapter VI. Manuscript Testimony Shewn to Be Overwhelmingly in Favour of These Verses.-Part I.
Chapter VII. Manuscript Testimony Shewn to Be Overwhelmingly in Favour of These Verses.-Part II.
Chapter VIII. The Purport of Ancient Scholia, and NOtes in MSS. on the Subject of These Verses, Shewn to Be the Reverse of Waht Is Commonly Supposed.
Chapter IX. Internal Evidence Demonstrated to Be the Very Reverse of Unfavourable to These Verses.
Chapter X. The Testimony of the Lectionaries Shewn to be Absolutely Decisive as to the Genuineness of These Verses.
Chapter XI. The Omission of These Twelve Verses in Certain Ancient Copies of the Gospels, Explained and Accounted for.
Chapter XII. General Review of the Question: Summary of the Evidence; and Conclusion of the Whole Subject.
Appendix (A). On the importance of attending to Patristic Citations of Scripture. The Correct Text of S. Luke ii. 14, established.
Appendix (B). Eusebius
Appendix (c). Proof that Hesychius is a copyist only in what he says concerning the end of S. Mark's Gospel.
Appendix (D). Some account of VICTOR OF ANTIOCH'S Commentary on S. Mark's Gospel; together with an enumeration of MSS. which contain Victor's Work.
Appendix (E). Text of the concluding Scholion of Victor of Antioch's Commentary on S. Mark's Gospel; in which Victor bears emphatic testimony to the genuineness of
Appendix (F). On the Relative antiquity of the Codex Vaticanus (B), and the Codex Sinaiticus (א).
Appendix (G). On the so-called
Appendix (H). On the INterpolation of the text of Codex B and Codex א of S. Matthew xxvii. 48 or 49.
Postscript.
L'Envoy
And this is what Burgon said in his introductory remarks:
Recent Editors of the New Testament insist that these “last Twelve Verses” are not genuine. The Critics, almost to a man, avow themselves of the same opinion. Popular Prejudice has been for a long time past warmly enlisted on the same side. I am as convinced as I am of my life, that the reverse is the truth. It is not even with me as it is with certain learned friends of mine, who, admitting the adversary’s premium, content themselves with denying the validity of his inference. However true it may be,—and it is true,—that from those premium the proposed conclusion does not follow, I yet venture to deny the correctness of those premisses altogether. I insist, on the contrary, that the Evidence relied on is untrustworthy,—untrustworthy in every particular...If I may be allowed to say so,—S. Mark’s last Twelve Verses shall no longer remain a subject of dispute among men. I am able to prove that this portion of the Gospel has been declared to be spurious on wholly mistaken grounds: and this ought in fairness to close the discussion. But I claim to have done more. I claim to have shewn, from considerations which have been hitherto overlooked, that its genuineness must needs be reckoned among the things that are absolutely certain.