If Jews were such honest keepers of God’s word, then why did they reject and kill true prophets although Deut. 13:1-5 & 18:20-22 tells criteria for judging true prophets?
It is always difficult to judge people’s motives. We can observe what they do and we can observe what they say. However, as Samuel told Jesse, the father of David, “People look at the outward appearance, but the Lord looks at the heart.” (1 Samuel 16:7). What we can see is that the Jewish scribes were absolutely fanatical about protecting and accurately copying the Hebrew scriptures. Was this because of their great love for God, or because of their simply being religious? Probably the motivation was very sincere and heart-felt by some, but more a matter of being religious, without the heart, by others. Like I said (and God agrees in 1 Samuel 16:7), it is difficult for us to judge the motives.
The fact remains that the Jews were fanatical in their preservation of the Hebrew Scripture. This is shown, both by the similarity between the Dead Sea Scrolls of about 100 BC and the Masoretic Text of about 900 BC. For example, the Isaiah scroll of the DSS is remarkably similar to the Masoretic text of 1000 years later. This shows the incredible attention to maintaining a reliable text of what we call the Old Testament.
Then there is the evidence from the rules laid down by the Masorete scribes. Here is an example of instructions recorded from the second half of the first Millennium:
Before even starting to copy the scrolls or codices, the scribe was required by the Masoretes to go through an elaborate ceremony. In order to preserve the integrity of the text, the Masorete scribes counted all the letters in the Old Testament. They kept track of such arcane details as the middle verse of the Pentateuch (Leviticus 8:7). They also found the middle verse of the entire Hebrew Bible (Jeremiah 6:7). They were aware of the middle word of the whole Old Testament, as well as the middle word of each book. They also kept record of the middle letter and verse of each book. Taking it to the extreme, they also counted the number of times each Hebrew letter appeared in each book and counted the number of verses which contained all the letters of the Hebrew alphabet. All this was intended to produce exact copies of the Scriptures. Imagine doing all this letter and word counting, and using it to check every copy of the entire Old Testament. And they did not have word processors!
Before the Masoretes: Talmud records instructions for making a copy in the first or second century AD:
A synagogue roll must be written on the skins of clean animals, prepared for the particular use of the synagogue by a Jew. These must be fastened together with strings taken from clean animals. Every skin must contain a certain number of columns, equal throughout the entire codex. The length of each column must not extend over less than forty-eight, or more than sixty lines; and the breadth must consist of thirty letters. The whole copy must be first lined; and if three words be written in it without a line, it is worthless. The ink should be black, neither red, green, nor any other color and be prepared according to a definite recipe. An authentic copy must be the exemplar, from which the transcriber ought not in the least deviate. No word or letter, not even a yod (a vowel mark), must be written from memory, the scribe not having looked at the codex before him…. Between every consonant the space of a hair or thread must intervene; between every word, the breadth of a narrow consonant; between every new section, the breadth of nine consonants; between every book, three lines. The fifth book of Moses must terminate exactly with a line, but the rest need not do so. Besides this, the copyist must sit in full Jewish dress, wash his whole body, not begin to write the name of God with a pen newly dipped in ink, and should a king address him while writing that name he must take no notice of him…. The rolls in which these regulations are not observed are condemned to be buried in the ground or burned; or they are banished to the schools, to be used as reading books.
 From Sir Frederic Kenyon, Our Bible and the Ancient Manuscripts (Harper and Brothers, New York, 1958) pp. 78-79.
So, we can see that the Jewish scribes certainly were honest keepers of God’s word.
The next question, then, is why did they reject and kill the true prophets, and, later, Jesus Christ himself. First of all, it was not necessarily the scribes and teachers who were wanting to kill the prophets. It was usually the kings, priests and other political leaders who had the prophets killed. In other words, those who maintained accurate Old Testament manuscripts were not necessarily the same ones who had the prophets killed. In the case of Jesus, it was the Sadducees, the Pharisees and the Sanhedrin who plotted to kill Jesus. The Scribes and teachers of the Law were not necessarily the ones most intent on killing Jesus (though they probably played a minor role).
In any case, it is surely possible to be adamant about the religious role of protecting the scriptures, and at the same time to have a hardened heart toward God. We see this all the time. Paul is a perfect example. He would have defenced the Torah to the death, but he also opposed the Jesus movement and wanted his followers killed. Often the prophets were killed for political, not religious reasons. They did not always support the kings–especially when they did what was evil. If we are modern-day “prophets”–calling sin sin and calling people to repentance, then many so-called Christians will not appreciate us one little bit!
This is human nature. We can be very religious, but also oppose what Jesus is trying to do. I believe that if Jesus came today, the ones who would most strongly oppose him are not atheists, but groups who call themselves Christian but who do not truly have a heart for Jesus.
Why do people reject Jesus? Because they do not like what he says. They do not want to repent of their sins, or they do not agree with what he says about God. He upsets their personal philosophy about God or their personal fleshly desires. It is complicated, but, to me, it is not at all surprising that Jesus’ strongest opponents were often the most stridently Jewish persons.
Besides, do not forget that MANY Jews did become Christians in the first century. Before the later hardening of the Jews, a very large proportion across the entire Jewish diaspora became Christians. Was it 15% or was it 35%, I do not claim to know, but I believe that as large a percentage of Jews became Christians as any other group in the first century. The lack of conversion of Jews is often exaggerated, and the percentage who converted is typically downplayed in my opinion (although I will admit I do not have hard numbers to quote, so I say this with a little bit of caution).