From what I understand, these are the best codices available.
Codex Sinaiticus is considered the only complete New Testament which was commissioned by Emperor Constantine still existing today.
Should we accept this codex as the source of truth for the New Testament? Who knows if this codex might be the most original Greek New Testament?
To answer your first question, we should not simply accept Sinaiticus as the source of the truth for the New Testament. It has great weight in debates from its age, but age is not the final arbiter in textual considerations.
Codex Sinaiticus was made in the 4th century on parchment using capital letters (a manuscript in all capitals is called an "uncial"). It was discovered in the 19th century, surpassing Vaticanus as the most complete manuscript. Codex Sinaiticus is considered by most textual scholars of the New Testament to be the best complete manuscript. It and Vaticanus are hypothesized to be part of Emperor Constantine's project, though this has never been conclusively proven either way.
It should be understood that "complete manuscript" when used by a textual critic does not necessarily mean 100% of it has survived. "Complete" is a technical term meaning that the manuscript has the beginning and end of the book in question. For example, a "complete copy of John" would be required to have John 1:1 and John 21:25 and substantial portions of those verses between.
Originally, Sinaiticus had the entire, Greek Old Testament (the Septuagint) as well as the complete New Testament. Only half of the Old Testament has survived, but the New Testament is complete in that all books are represented while only a few passages and verses are missing due to pages missing, holes in the pages, or scribal exclusions.
While it is significant that Sinaiticus is the oldest complete manuscript, it is not the oldest manuscript. There are pieces of different books that are much older. P52 (a piece of John's Gospel) has been dated to AD 117 at the oldest based on the style of hand writing (though some argue for a date into the second half of the first century). Possibly even older, is a recently discovered fragment of Mark that some are saying they are "certain" it is from the first century. There are also copies of entire books that are older than Sinaiticus. For example, P46 contains all of Hebrews, Ephesians, Philippians, Galatians, and Colossians, and virtually all of 1 and 2 Corinthians. It is dated between AD 175 and 225. Likeiwse, P66 (from roughly the same time period) contains most of John, but the ending is missing so it is not considered complete.
For the Gospels, Sinaiticus is generally considered among scholars as the second most reliable witness of the text (after Vaticanus); in the Acts of the Apostles, its text is equal to that of Vaticanus; in the Epistles, Sinaiticus is the most reliable witness of the text. In the Book of Revelation, however, its text is corrupted and is considered of poor quality, and inferior to the texts of Codex Alexandrinus, Papyrus 47, and even some minuscule manuscripts in this place (for example, Minuscule 2053, 2062).
However, even in the epistles, where it is considered the most reliable, it is not merely accepted. Textual critics and scholars will compare many manuscripts to determine the original text. By studying the copies and copying styles, they have put together a list of errors that scribes were likely to make and they can compare manuscripts to see which wording is more likely to be original.
You may enjoy this article on textual criticism in action.Tweet