This is a common objection today which is sometimes the result of misinformation of reading from poor quality sources such as popular journalism, certain you-tube videos, and television documentaries which are constantly appeasing a sensationalist audience and take a pragmatic and non-scholarly approach. The critical issues are accuracy of the text, the number of texts available, the transmission of the same and what is and is not included.
Accuracy and Transmission
How can a text be passed on accurately over hundreds and even thousands of years? When Jewish scribes copied their texts, they would do so paying careful attention to the columns and rows and count the number of words and check them ever so finely. If a mistake were found that scroll would be jettisoned. Furthermore large numbers of scrolls were produced.
But surely mistakes appear over a process of time since materials wear out and copies of copies have to be made? Until the last century the earliest copy we had of the Tanakh (Old testament) was the Masoretic Text, around 1000AD. With such a long time between that and the Septuagint, written by the 70 Jewish scholars in Greek around 300BC one would expect more discrepancies. The Dead Sea Scrolls finding in 1948 was the most Biblical significant archaeological discovery of the last century. Sure there are minor variations with regard to punctuation and some words, but the consistency of the respective texts and the essential meaning is remarkable.
Also the number of texts speak for themselves. There are over 5000 early Greek New Testament texts plus writings in Coptic, Syriac, and other languages. If you compared that with the Iliad, Xenophon, Tacitus, or any other notable ancient historical work then the Bible has overwhelmingly more copies than the other texts. The earliest New Testament fragment is in the John Rylands library in the university of Manchester and is from the first century.
What was and what was not included in the canon?
The Apocrypha fits between the Old and New Testament so why is it not considered part of the official canon? Interestingly neither the Lord Jesus nor the New Testament quotes from it, yet they both quote extensively from the Tanakh (Old Testament). However, it does provide us with valuable history such as the Maccabean revolt and the first Hanukkah (2 Maccabees 10:1-8). Josephus refers to this as the festival of lights in Antiquities 12:325 and Yeshua (Jesus) also kept the feast of dedication (Hanukkah-John 10:22). In short, the Apocrypha is useful as a historical source but not for doctrine.
So what about the New Testament canon? Often arguments are presented that many were left out and that it took until AD325 and the council of Nicaea to decide! Firstly the Gnostic Gospels were written much later than the books in the New Testament Canon, all in the first century or later and far fewer copies were produced. Even if the Gospel of Judas was written by him, which is astonishingly unlikely since he immediately hung himself after betraying the Lord, would someone who was unfaithful with the treasury money and who betrayed the Lord stand up as a credible witness in court? The Gnostic gospel writers were not eye-witnesses. Secondly early church fathers confirm which books were considered Scripture and quoted as such. The Council of Nicaea simply confirmed which Scriptures were already considered inspired Scriptures.
There is also an important internal consistency between the books within the Bible itself which fit together perfectly. Noticeably Peter writes that he considers all of Paul’s writings as part of Scripture (2 Peter 3:14-16). The New Testament books were written mostly by the apostles, and some writings by close associates of the apostles. The apostles were people who had seen the risen Lord Jesus and were had miraculous signs accompanying their ministry, as they were sent by God.
All Scripture is inspired and although written by humans, the Bible is God-breathed (inspired by the Holy Spirit- 2 Timothy 3:16; 2 Peter 1:20-21).
One more thing…
For those reading this that are Theists and believe that there is a single sovereign God; is not the Creator of the heavens and the earth, the One who sustains the Universe able to preserve His book? We are expressly warned neither to take away or adds to God’s word (Deuteronomy 4:2; 12:32; Proverbs 30:5-6; Matthew 5:17-18; Revelation 22:18). The Bible has been preserved as Isaiah wrote, “The grass withers, the flower fades, but the word of God stands forever (Isaiah 40:8).”