Knowledge is very important in all fields, but derives from different sources. One can know science by observation and experimentation since physical phenomena are perceivable by the senses and repeatable. Historical events, however, are not repeatable and so persons of later eras must rely on written records and on artifacts from earlier eras if they wish to understand them. The best understanding usually comes from a combination of sources.
The first major source of information about the intertestamental period is the Bible. Culture doesn’t change overnight, and so many of the lifestyles and opinions found in the late Old Testament persisted until and even through the intertestamental period. Lifestyles and practices we find in the New Testament are likely to be similar to those preceding it, especially in an age when change was far slower than in modern times. One can trace the postexilic reforms of Ezra and Nehemiah to the teachings of the Hasidim and then to the Pharisees of Jesus’ day.