Mystery of the Old Testament in the Bible
Why is the Old Testament attached to the New Testament as part of the Christian Bible?
The reason must be that when the Bible was compiled and established at the end of the fourth century by sorting out various documents related to work of early Christians and so on, Judaist Christians still accounted for a considerable percentage.
Christianity must not have been separately taught but along with Judaism in synagogues mostly in cities where many diaspora Jewry lived. In the Roman Empire, Christianity must have spread though synagogues in Rome and other cities to other Roman citizens who associated with those Judaists.
If at the end of the 4th century, a majority of Christians in the Roman Empire had been others than Judaists, the Old Testament books should not have been adopted for the Christian Bible, or at least not all of the books of the Old Testament books should have been adopted.
So, till the 5th century, in the Roman Empire, there must have been a considerable number of Judaists who were also Christians. They must have joined both the Christian churches and the Judaists synagogues. And, they must have claimed to adopt the Old Testament for the compilation of the Christian Bible.
However, at the same time, the Great Barbarian Invasion started, having been triggered by the Huns moving from the central Asia to east Europe. As if having been pushed by the Huns, various tribes of northern and eastern Europeans and other races invaded the territory of the Roman Empire, putting the end to the rule of Rome in the late 5th century.
I suppose in the period between the end of the 4th century and the end of the 5th century when the Western Roman Empire collapsed, the separation between Judaists and Judaist Christians occurred. After this period, no more Judaists came to become Judaist Christians. And, Christians stopped inviting Judaists to Christian churches.
But as the Christian Bible was compiled and established at the end of the 4th century, the existence of Judaist Christians was reflected in the adoption of the Old Testament for part of the Bible.
But why and how did the Great Barbarian Invasion contribute to the complete separation between Christians and Judaists?
Probably, Christians in the Roman Empire tried to save some legacy of the Empire, thus concentrating their activities around Rome, which must have led to the establishment of the Vatican. On the other hand, Judaists did not fight against northern and eastern invaders but rather welcomed them as they could be free from the rule by Romans who had once occupied and destroyed Jerusalem.
Therefore, even today it must be easier for Judaists to convert to Christianity as the Bible includes the Old Testament that is a collection of Judaism books.
Anyway, the Old Testament in the Bible reminds us that Christianity started among Judaists and was propagated by Judaists.
https://en.wikipedia.org/wiki/Migration_Period
http://www.religionandspirituality.com/view/post/1360877079145/
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Mat 12:25 And Jesus knew their thoughts, and said unto them, Every kingdom divided against itself is brought to desolation; and every city or house divided against itself shall not stand:
Mar 12:25 For when they shall rise from the dead, they neither marry, nor are given in marriage; but are as the angels which are in heaven.
Luk 12:25 And which of you with taking thought can add to his stature one cubit?
Joh 12:25 He that loveth his life shall lose it; and he that hateth his life in this world shall keep it unto life eternal.